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]
Merge pull request #12524 from kirs/patch-1

Prepare generated Gemfile for Capistrano 3