As with Rails 4.1, Rails 4.2 features only minor changes to the config/application.rb file.

          require File.expand_path('../boot', __FILE__)

          require 'rails/all'

          # Require the gems listed in Gemfile, including any gems
          # you've limited to :test, :development, or :production.
          Bundler.require(*Rails.groups)

          module Rails42ExampleApp
            class Application < Rails::Application
              # Settings in config/environments/* take precedence over those specified here.
              # Application configuration should go into files in config/initializers
              # -- all .rb files in that directory are automatically loaded.

              # Set Time.zone default to the specified zone and make Active Record auto-convert to this zone.
              # Run "rake -D time" for a list of tasks for finding time zone names. Default is UTC.
              # config.time_zone = 'Central Time (US & Canada)'

              # The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded.
              # config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}').to_s]
              # config.i18n.default_locale = :de

              # Do not swallow errors in after_commit/after_rollback callbacks.
              config.active_record.raise_in_transactional_callbacks = true
            end
          end

          

Once again, only a few minor changes for the config/environments/development.rb file as well.

          Rails.application.configure do
            # Settings specified here will take precedence over those in config/application.rb.

            # In the development environment your application's code is reloaded on
            # every request. This slows down response time but is perfect for development
            # since you don't have to restart the web server when you make code changes.
            config.cache_classes = false

            # Do not eager load code on boot.
            config.eager_load = false

            # Show full error reports and disable caching.
            config.consider_all_requests_local       = true
            config.action_controller.perform_caching = false

            # Don't care if the mailer can't send.
            config.action_mailer.raise_delivery_errors = false

            # Print deprecation notices to the Rails logger.
            config.active_support.deprecation = :log

            # Raise an error on page load if there are pending migrations.
            config.active_record.migration_error = :page_load

            # Debug mode disables concatenation and preprocessing of assets.
            # This option may cause significant delays in view rendering with a large
            # number of complex assets.
            config.assets.debug = true

            # Asset digests allow you to set far-future HTTP expiration dates on all assets,
            # yet still be able to expire them through the digest params.
            config.assets.digest = true

            # Adds additional error checking when serving assets at runtime.
            # Checks for improperly declared sprockets dependencies.
            # Raises helpful error messages.
            config.assets.raise_runtime_errors = true

            # Raises error for missing translations
            # config.action_view.raise_on_missing_translations = true
          end

          

One important notable change here. config.serve_static_assets has been changed to config.serve_static_files. Remember this!

          Rails.application.configure do
            # Settings specified here will take precedence over those in config/application.rb.

            # Code is not reloaded between requests.
            config.cache_classes = true

            # Eager load code on boot. This eager loads most of Rails and
            # your application in memory, allowing both threaded web servers
            # and those relying on copy on write to perform better.
            # Rake tasks automatically ignore this option for performance.
            config.eager_load = true

            # Full error reports are disabled and caching is turned on.
            config.consider_all_requests_local       = false
            config.action_controller.perform_caching = true

            # Enable Rack::Cache to put a simple HTTP cache in front of your application
            # Add `rack-cache` to your Gemfile before enabling this.
            # For large-scale production use, consider using a caching reverse proxy like
            # NGINX, varnish or squid.
            # config.action_dispatch.rack_cache = true

            # Disable serving static files from the `/public` folder by default since
            # Apache or NGINX already handles this.
            config.serve_static_files = ENV['RAILS_SERVE_STATIC_FILES'].present?

            # Compress JavaScripts and CSS.
            config.assets.js_compressor = :uglifier
            # config.assets.css_compressor = :sass

            # Do not fallback to assets pipeline if a precompiled asset is missed.
            config.assets.compile = false

            # Asset digests allow you to set far-future HTTP expiration dates on all assets,
            # yet still be able to expire them through the digest params.
            config.assets.digest = true

            # `config.assets.precompile` and `config.assets.version` have moved to config/initializers/assets.rb

            # Specifies the header that your server uses for sending files.
            # config.action_dispatch.x_sendfile_header = 'X-Sendfile' # for Apache
            # config.action_dispatch.x_sendfile_header = 'X-Accel-Redirect' # for NGINX

            # Force all access to the app over SSL, use Strict-Transport-Security, and use secure cookies.
            # config.force_ssl = true

            # Use the lowest log level to ensure availability of diagnostic information
            # when problems arise.
            config.log_level = :debug

            # Prepend all log lines with the following tags.
            # config.log_tags = [ :subdomain, :uuid ]

            # Use a different logger for distributed setups.
            # config.logger = ActiveSupport::TaggedLogging.new(SyslogLogger.new)

            # Use a different cache store in production.
            # config.cache_store = :mem_cache_store

            # Enable serving of images, stylesheets, and JavaScripts from an asset server.
            # config.action_controller.asset_host = 'http://assets.example.com'

            # Ignore bad email addresses and do not raise email delivery errors.
            # Set this to true and configure the email server for immediate delivery to raise delivery errors.
            # config.action_mailer.raise_delivery_errors = false

            # Enable locale fallbacks for I18n (makes lookups for any locale fall back to
            # the I18n.default_locale when a translation cannot be found).
            config.i18n.fallbacks = true

            # Send deprecation notices to registered listeners.
            config.active_support.deprecation = :notify

            # Use default logging formatter so that PID and timestamp are not suppressed.
            config.log_formatter = ::Logger::Formatter.new

            # Do not dump schema after migrations.
            config.active_record.dump_schema_after_migration = false
          end

          

A few minor changes here. Also again note that config.serve_static_assets has been changed to config.serve_static_files.

            Rails.application.configure do
              # Settings specified here will take precedence over those in config/application.rb.

              # The test environment is used exclusively to run your application's
              # test suite. You never need to work with it otherwise. Remember that
              # your test database is "scratch space" for the test suite and is wiped
              # and recreated between test runs. Don't rely on the data there!
              config.cache_classes = true

              # Do not eager load code on boot. This avoids loading your whole application
              # just for the purpose of running a single test. If you are using a tool that
              # preloads Rails for running tests, you may have to set it to true.
              config.eager_load = false

              # Configure static file server for tests with Cache-Control for performance.
              config.serve_static_files   = true
              config.static_cache_control = 'public, max-age=3600'

              # Show full error reports and disable caching.
              config.consider_all_requests_local       = true
              config.action_controller.perform_caching = false

              # Raise exceptions instead of rendering exception templates.
              config.action_dispatch.show_exceptions = false

              # Disable request forgery protection in test environment.
              config.action_controller.allow_forgery_protection = false

              # Tell Action Mailer not to deliver emails to the real world.
              # The :test delivery method accumulates sent emails in the
              # ActionMailer::Base.deliveries array.
              config.action_mailer.delivery_method = :test

              # Randomize the order test cases are executed.
              config.active_support.test_order = :random

              # Print deprecation notices to the stderr.
              config.active_support.deprecation = :stderr

              # Raises error for missing translations
              # config.action_view.raise_on_missing_translations = true
            end

          

For the people that still haven't upgraded.

          # Be sure to restart your server when you modify this file.

          # Your secret key is used for verifying the integrity of signed cookies.
          # If you change this key, all old signed cookies will become invalid!

          # Make sure the secret is at least 30 characters and all random,
          # no regular words or you'll be exposed to dictionary attacks.
          # You can use `rake secret` to generate a secure secret key.

          # Make sure the secrets in this file are kept private
          # if you're sharing your code publicly.

          development:
            secret_key_base: <A SECRET KEY>

          test:
            secret_key_base: <A SECRET KEY>

          # Do not keep production secrets in the repository,
          # instead read values from the environment.
          production:
            secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>