HHVM nightly is core-dumping again skip it for now.

.travis.yml

@@ -28,6 +28,7 @@ matrix:
       env: HHVM=1 DB=mysql db_dsn='mysql://travis@0.0.0.0/cakephp_test'
   allow_failures:
     - php: 7.0
+    - php: hhvm-nightly
 
 before_script:
   - composer self-update