I did therefore create a buildpack which basically downloads JRuby, install Bundler and run
bundle install. After that it behaves exactly like a normal Heroku Ruby application!
Read more about the JRuby buildpack at github.com/jruby/heroku-buildpack-jruby
Take a look at an example application here: github.com/carlhoerberg/heroku-jruby-example
The buildpack is moved and now maintained under the JRuby organization: github.com/jruby/heroku-buildpack-jruby
If you already have created an application with the old buildpack url you can just update the environment variable to the new url: