Require HHVM to pass now.

https://github.com/facebook/hhvm/pull/1860

.travis.yml

@@ -14,7 +14,3 @@ php:
   - 5.5
   - 5.6
   - hhvm
-
-matrix:
-  allow_failures:
-    - php: hhvm # See https://github.com/facebook/hhvm/pull/1860