Add ruby-head to allowed failures, since the version on travis is outdated

It's due to bug reverted in ruby/ruby@0516a33

Author: Zachary Scott

.travis.yml

@@ -7,7 +7,7 @@ before_script:
 language: ruby
 notifications:
   email:
-  - [email protected]
+  - [email protected]
 rvm:
 - 1.8.7
 - 1.9.3
@@ -21,3 +21,4 @@ matrix:
   allow_failures:
     - rvm: 1.8.7
     - rvm: rbx-2
+    - rvm: ruby-head