Fix HHVM build for now again and ignore future HHVM build errors

Author: clue

.travis.yml

@@ -7,9 +7,12 @@ php:
   - 7.0
   - 7.1
   - nightly
-  - hhvm
 
+# also test against HHVM, but require "trusty" and ignore errors
 matrix:
+  include:
+    - php: hhvm
+      dist: trusty
   allow_failures:
     - php: nightly
     - php: hhvm