Drop support for Ruby 2.0 as it's EOLd

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

.travis.yml

 rvm:
 rvm:
-  - 2.0
   - 2.1
   - 2.1
   - 2.2
   - 2.2
 
 

NEWS

 master:
 master:
 
 
+* Drops support to end-of-life'd ruby 2.0.
 * Improvement: Paperclip now supports aws-sdk v2
 * Improvement: Paperclip now supports aws-sdk v2
   @betesh, @davetchen,
   @betesh, @davetchen,
   https://github.com/thoughtbot/paperclip/pull/1903
   https://github.com/thoughtbot/paperclip/pull/1903