Drop HHVM support, add PHP 7.1 testing.

Author: bojanz

.travis.yml

@@ -2,10 +2,10 @@ sudo: false
 language: php
 
 php:
-  - 5.5
-  - 5.6
+  - 7.1
   - 7.0
-  - hhvm
+  - 5.6
+  - 5.5
 
 install:
   - composer self-update