Update Travis build configuration

* Change `rbx` to `rbx-2`
* Mark `rbx-2` as allow-failure again

.travis.yml

 rvm:
 rvm:
   - 1.9.3
   - 1.9.3
   - jruby-19mode
   - jruby-19mode
-  - rbx
+  - rbx-2
   - 2.0.0
   - 2.0.0
   - 2.1.1
   - 2.1.1
 
 
@@ -20,4 +20,4 @@ matrix:
   fast_finish: true
   fast_finish: true
   allow_failures:
   allow_failures:
     - rvm: jruby-19mode
     - rvm: jruby-19mode
-    - rvm: rbx-19mode
+    - rvm: rbx-2