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

Author: clue

.travis.yml

@@ -7,7 +7,14 @@ php:
   - 5.6
   - 7.0
   - 7.1
-  - hhvm
+
+# also test against HHVM, but require "trusty" and ignore errors
+matrix:
+  include:
+    - php: hhvm
+      dist: trusty
+  allow_failures:
+    - php: hhvm
 
 sudo: false