Replaced hhvm with php 7.1 env at travis

Author: norberttech

.travis.yml

@@ -15,7 +15,7 @@ matrix:
     - php: 5.5
     - php: 5.6
     - php: 7.0
-    - php: hhvm
+    - php: 7.1
 
 before_install:
   - composer self-update