Updated the list of ruby versions for TravisCI to test
Updated the list of ruby versions for TravisCI to test

There’s no need to test on versions 2.0, 2.1, or 2.2 anymore because they’re all EOL. Ruby >= 2.5 can’t be added to the list until the mathn dependency is removed.

- https://www.ruby-lang.org/en/news/2016/02/24/support-plan-of-ruby-2-0-0-and-2-1/
- https://www.ruby-lang.org/en/news/2017/04/01/support-of-ruby-2-1-has-ended/
- https://www.ruby-lang.org/en/news/2018/06/20/support-of-ruby-2-2-has-ended/

bfoz/geometry