minor #537 Use the newer stack for HHVM on Tracis CI (bocharsky-bw)

This PR was squashed before being merged into the master branch (closes #537).

Discussion
----------

Use the newer stack for HHVM on Tracis CI

Commits
-------

8bdcc66 Use the newer stack for HHVM on Tracis CI

Author: javiereguiluz

.travis.yml

@@ -12,7 +12,11 @@ matrix:
     - php: 5.6
     - php: 7.0
     - php: 7.1
+    # Use the newer stack for HHVM as HHVM does not support Precise anymore since a long time and so Precise has an outdated version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
+      sudo: required
+      dist: trusty
+      group: edge
   allow_failures:
     - php: hhvm