Merge pull request #19 from e2/drop_older_rubies

drop support for Ruby < 2.2.5

Author: e2

.travis.yml

@@ -2,12 +2,8 @@ sudo: false
 language: ruby
 bundler_args: --without development
 rvm:
-  - 2.0.0
-  - 2.1.9
-  - 2.2.4
   - 2.2.5
   - 2.3.1
-  - jruby-9.0.5.0
   - jruby-9.1.2.0
-before_install: gem install bundler -v 1.12.1
+before_install: gem install bundler -v 1.12.5
 cache: bundler