Change effective on 20 February 2018
The version of rubygems bundled with Ruby 2.5.0 was updated to 2.7.6 on heroku-16 and cedar-14 stacks. You can see the current rubygems version associated with on the Ruby Version Support page. This applies these security fixes.
To see the current version of rubygems installed for your app you can run:
$ heroku run bash
$ gem -v 2.7.6