Use newest revision of Ruby 2.1 & 2.2 on Travis

By not specifying patch level for Ruby 2.1 & 2.2, Travis should just select most recent version available.
Also added Rubinius (in most recent version) build target.

Author: Tobias Haagen Michaelsen

.travis.yml

@@ -3,8 +3,9 @@ script: rake
 rvm:
   - 1.9.3
   - 2.0.0
-  - 2.1.0
-  - 2.2.0
+  - "2.1"
+  - "2.2"
   - ruby-head
   - jruby-head
   - jruby-19mode
+  - rbx