Drop support for 2.0.0 as it is no longer supported

* See https://www.ruby-lang.org/en/news/2016/02/24/support-plan-of-ruby-2-0-0-and-2-1/

Author: eregon

.travis.yml

@@ -1,7 +1,6 @@
 sudo: false
 language: ruby
 rvm:
-  - 2.0.0-p648
   - 2.1.8
   - 2.2.4
   - ruby-head