Change effective on 01 March 2017
Since Ruby applications were upgraded to Bundler 1.13.7 the Ruby version is now stored in the Gemfile.lock. This caused problems with JRuby builds as Bundler includes the patchlevel in the
Gemfile.lock even though for Heroku it is not significant.
This is the same behavior that Heroku had prior to upgrading to Bundler 1.13.7. You can view the Supported Ruby Runtimes in DevCenter.