Add Ruby 2.2 2.1 2.0 to travis

Author: asmsuechan

.travis.yml

@@ -1,6 +1,9 @@
 sudo: false
 language: ruby
 rvm:
-  - 2.3.1
-before_install: gem install bundler -v 1.13.1
+  - 2.3.0
+  - 2.2.0
+  - 2.1.0
+  - 2.0.0
+before_install: gem install bundler
 install: bundle install