Merge branch 'remove-rails5-gemfile' into 'master'

Remove Gemfile.rails5

See merge request gitlab-org/gitlab-ce!23109

Gemfile.rails5

-# BUNDLE_GEMFILE=Gemfile.rails5 bundle install
-
-ENV["RAILS5"] = "true"
-
-gemfile = File.expand_path("../Gemfile", __FILE__)
-
-eval(File.read(gemfile), nil, gemfile)