Merge branch 'blackst0ne-add-rails5-method' into 'master'

[Rails5] Add `Gitlab.rails5?` method

See merge request gitlab-org/gitlab-ce!17660

config/application.rb

@@ -194,4 +194,10 @@ module Gitlab
       Gitlab::Routing.add_helpers(MilestonesRoutingHelper)
     end
   end
+
+  # This method is used for smooth upgrading from the current Rails 4.x to Rails 5.0.
+  # https://gitlab.com/gitlab-org/gitlab-ce/issues/14286
+  def self.rails5?
+    ENV["RAILS5"].in?(%w[1 true])
+  end
 end