Test HHVM but allow failures.

Author: frankmayer

.travis.yml

@@ -1,12 +1,15 @@
 language: php
+
 dist: trusty
 
 matrix:
+  fast_finish: true
   include:
     - php: '5.6'
     - php: '7.0'
     - php: '7.1'
-  fast_finish: true
+  allow_failures:
+    - php: hhvm
 
 before_script:
   - chmod 777 ./tests/travis/setup_arangodb.sh