Prepare generated Gemfile for Capistrano 3

Capistrano 3 is now rails-less, so we need to encourage developers to use capistrano-rails gem.

More details: http://www.capistranorb.com/documentation/frameworks/ruby-on-rails/

[ci skip]

railties/lib/rails/generators/rails/app/templates/Gemfile

@@ -25,7 +25,7 @@ end
 # gem 'unicorn'
 
 # Use Capistrano for deployment
-# gem 'capistrano', group: :development
+# gem 'capistrano-rails', group: :development
 
 <% unless defined?(JRUBY_VERSION) -%>
 # Use debugger