Change effective on 28 August 2017
The version of rubygems bundled with Ruby 2.4.1 was updated to 2.6.13 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 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux] $ gem -v 2.6.13