`ruby_21` is not a valid platform` with older versions of Bundler
Using :platforms in a Gemfile with newer Ruby versions is incompatible with older Bundler versions that we use on a number of packaged OSes. The Gemfile should use RUBY_VERSION instead of :platforms to restrict by Ruby version.
Oct 20 07:47:01 localhost systemd: Starting Foreman Proxy... Oct 20 07:47:01 localhost smart-proxy: /usr/share/foreman-proxy/Gemfile.in:9:in `instance_eval': `ruby_21` is not a valid platform. The available options are: [:ruby, :ruby_18, :ruby_19, :ruby_20, :mri, :mri_18, :mri_19, :mri_20, :rbx, :jruby, :mswin, :mingw, :mingw_18, :mingw_19, :mingw_20] (Bundler::GemfileError)