Gemfile: Use :ruby to denote platform-with-C-exts
Gemfile: Use :ruby to denote platform-with-C-exts

This change is a follow-up to e168cb5d49d76bbfb9d719c610bccef69747b62b

Thanks to eregon for noting that :mri is distinct from :ruby.

sinatra/sinatra,sinatra/sinatra,ryanshaw/sinatra,ryanshaw/sinatra