Change effective on 30 August 2017
The version of rubygems bundled with Ruby 2.3.4 was updated to 22.214.171.124 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 126.96.36.199