[travis] Specify min/max/hhvm php runtimes

Author: yukoff

.travis.yml

@@ -1,6 +1,11 @@
 sudo: false
 language: php
 
+php:
+  - 5.6
+  - nightly
+  - hhvm
+
 before_script:
   - composer self-update
   - if [ "$SF_EVT_DISPATCHER_VERSION" != "" ]; then composer require --no-update symfony/event-dispatcher:${SF_EVT_DISPATCHER_VERSION}; fi;