Change effective on 30 August 2017
The version of rubygems bundled with Ruby 2.3.4 was updated to 220.127.116.11 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 2.3.4p301 (2017-03-30 revision 58214) [x86_64-linux] $ gem -v 18.104.22.168