Change effective on 19 October 2017
Ruby v2.5.0 preview1 is now available on Heroku. To run your app using this preview version of Ruby, add the following
ruby directive to your Gemfile:
Note: This version of Ruby is not suitable for production applications. However, it can be used to test that your application is ready for December release of Ruby 2.5.0 and to provide feedback to the Ruby core team.