No longer test the gem on Ruby 2.1
No longer test the gem on Ruby 2.1

Official support for Ruby 2.1.x
[ended on 1st April 2017](https://www.ruby-lang.org/en/news/2017/04/01/support-of-ruby-2-1-has-ended/).
The gem may continue to work, but this is not guaranteed or to be
expected.

ejholmes/restforce,restforce/restforce,ejholmes/restforce,restforce/restforce