title: How to regenerate config/application.rb in Rails 4.2?
body: <p>Does Rails 4.2 provide a single command that regenerates <code>config/application.rb</code> in an existing application?</p>
  
  <p>The reason I ask is imagine a Rails app is <code>rails new</code>-ed without the <code>--skip-test-unit</code> option. </p>
  
  <p>Then at a much later date, this app is switched to RSpec. How can <code>config/application.rb</code> be regenerated as if the <code>--skip-test-unit</code> option had been supplied originally to <code>rails new ...</code>?</p>
  
  <p>All this would effectively do is change the <code>require</code> statements near the top of <code>config/application.rb</code> file from:</p>
  
  <pre><code>require 'rails/all'
  </code></pre>
  
  <p>to:</p>
  
  <pre><code>require "rails"
  # Pick the frameworks you want:
  require "active_model/railtie"
  require "active_job/railtie"
  require "active_record/railtie"
  require "action_controller/railtie"
  require "action_mailer/railtie"
  require "action_view/railtie"
  require "sprockets/railtie"
  # require "rails/test_unit/railtie"
  </code></pre>