Added hhvm to travis.yml

.travis.yml

@@ -5,6 +5,11 @@ php:
   - 5.3
   - 5.4
   - 5.5
+  - hhvm
+
+matrix:
+  allow_failures:
+    - php: hhvm
 
 before_script:
   - composer install --dev --prefer-source