Try a more recent rubinius 2.x

* Rubinius 2.0 is causing multiple untraceable errors
* One of the lines from the stacktrace gives same error as
  rubinius/rubinius#3013
* Apparently indicates broken Rubinius install
* Try a different version, see if less flaky

.travis.yml

@@ -6,7 +6,7 @@ rvm:
   - 2.3.0
   - ruby-head
   - jruby
-  - rbx-2
+  - rbx-2.11
 
 matrix:
   allow_failures: