Remove allowed failure, switch to testing newest 2.1

Because of Ruby's new versioning scheme, testing 2.1.0 and 2.1.1
separately is overkill (like testing multiple patch levels of earlier
Ruby versions).

.travis.yml

@@ -2,8 +2,7 @@ rvm:
   - 1.9.2
   - 1.9.3
   - 2.0.0
-  - 2.1.0
-  - 2.1.1
+  - 2.1
   - ruby-head
   - rbx
   - jruby
@@ -14,7 +13,6 @@ script:
 
 matrix:
   allow_failures:
-    - rvm: 2.1.0
     - rvm: ruby-head
     - rvm: jruby-head