Do not specify HHVM version

Author: bocharsky-bw

.travis.yml

@@ -15,7 +15,7 @@ matrix:
     - php: hhvm
   allow_failures:
     # Use the newer stack for HHVM as HHVM does not support Precise anymore since a long time and so Precise has an outdated version
-    - php: hhvm-3.18
+    - php: hhvm
       sudo: required
       dist: trusty
       group: edge