only test supported rubies and do not test twice

Author: mwrock

Diff for: .travis.yml

@@ -1,10 +1,14 @@
 language: ruby
 rvm:
-  - 1.9.3
-  - 1.9.2
-  - 2.0.0
-  - rbx-19mode
-  - rbx-18mode
-  - jruby-19mode
+  - 2.1.9
+  - 2.2.0
+  - 2.3.1
+  - rbx-2.2.2
+  - jruby-head
 before_install:
   - gem update bundler
+
+# This prevents testing branches that are created just for PRs
+branches:
+  only:
+  - master