[travis] Add min/max/hhvm php runtimes to matrix

Author: yukoff

.travis.yml

@@ -2,7 +2,9 @@ sudo: false
 language: php
 
 php:
+  - 5.5
   - nightly
+  - hhvm
 
 cache:
   directories:
@@ -59,4 +61,5 @@ matrix:
 
   allow_failures:
     - php: nightly
+    - php: hhvm
   fast_finish: true