Remove HHVM as ZMQ does not seem to be compatible

.travis.yml

@@ -8,16 +8,14 @@ php:
   - '5.5'
   - '5.6'
   - '7.0'
-  - hhvm
 
 matrix:
   fast_finish: true
   allow_failures:
     - php: '7.0'
-    - php: hhvm
 
 before_script:
-  - if [[ $TRAVIS_PHP_VERSION != hhvm ]]; then phpenv config-rm xdebug.ini; fi;
+  - phpenv config-rm xdebug.ini
   - echo "extension=zmq.so" >> `php --ini | grep "Loaded Configuration" | sed -e "s|.*:\s*||"`
   - composer self-update
   - composer install --prefer-source