added dist:trusty for hhvm in travis-ci file

Author: modiamir

.travis.yml

@@ -30,6 +30,8 @@ matrix:
           env: SYMFONY_VERSION=3.0.x
         - php: 7.0
           env: SYMFONY_VERSION=3.1.x
+        - php: hhvm
+          dist: trusty
 
 before_install:
   - if [ "$TRAVIS_PHP_VERSION" != "hhvm" ]; then echo 'memory_limit=-1' >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini; fi