Update .travis.yml

Author: dbu

.travis.yml

@@ -4,7 +4,6 @@ php:
   - 5.6
   - 7.0
   - 7.1
-  - hhvm
 
 env:
   - PACKAGE_VERSION=high
@@ -15,6 +14,8 @@ matrix:
   include:
     - php: 5.6
       env: PACKAGE_VERSION=low
+    - php: hhvm
+      dist: trusty
 
 before_script:
   - composer selfupdate