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

Author: clue

.travis.yml

@@ -1,9 +1,20 @@
 language: php
+
 php:
   - 5.3
   - 5.6
   - hhvm
+
+# also test against HHVM, but require "trusty" and ignore errors
+matrix:
+  include:
+    - php: hhvm
+      dist: trusty
+  allow_failures:
+    - php: hhvm
+
 install:
   - composer install --prefer-source --no-interaction
+
 script:
   - phpunit --coverage-text