Remove hhvm from the travis build matrix

Author: rikbruil

.travis.yml

@@ -16,7 +16,7 @@ before_install:
   - travis_retry composer self-update
 
 install:
-  - travis_retry composer install --no-interaction --prefer-source
+  - travis_retry composer install --no-interaction
 
 before_script:
   - mkdir -p build/logs