Use hhvm 3 explicitly

sabberworm · sabberworm · commit aecec873cfd3 · 2019-02-22T08:58:40.000+01:00
diff --git a/.travis.yml b/.travis.yml
@@ -7,7 +7,7 @@ php:
     - "7.1"
     - "7.2"
     - "7.3"
-    - hhvm
+    - hhvm-3.30.3
 sudo: false
 dist: trusty
 matrix:
