Rubies 2.2.9, 2.3.6, and 2.4.3 are now available
Change effective on 14 December 2017
Ruby v2.4.3, v2.3.6, v2.2.9 are now available on Heroku. To run your app using a newer version of Ruby, use ruby directive in your Gemfile. For instance, to use Ruby v2.4.3:
ruby "2.4.3"