[TASK] Add HHVM as build environment on Travis-CI and allow it to fail

Closes: #113

Author: Stefano Kowalke

.travis.yml

@@ -5,10 +5,15 @@ php:
   - 5.4
   - 5.5
   - 5.6
+  - hhvm
 
 env:
   - DB=mysql TYPO3=coreapi/develop INTEGRATION=master COVERAGE=0
 
+matrix:
+  allow_failures:
+    - php: hhvm
+
 services:
   - memcached
   - redis-server