Change effective on 30 August 2017
The version of rubygems bundled with Ruby 2.2.7 was updated to 18.104.22.168 on heroku-16 and cedar-14 stacks. You can see the current rubygems version associated with on the Ruby Version Support page.
To see the current version of rubygems installed for your app you can run:
$ heroku run bash
$ ruby -v ruby 2.2.7p470 (2017-03-28 revision 58194) [x86_64-linux] $ gem -v 22.214.171.124