Ignore unrelated HHVM build errors on Travis

Author: clue

.travis.yml

@@ -8,7 +8,7 @@ php:
   - 7.0
   - 7.1
   - 7.2
-  - hhvm
+  - hhvm # ignore errors, see below
 
 # lock distro so new future defaults will not break the build
 dist: trusty
@@ -17,6 +17,8 @@ matrix:
   include:
     - php: 5.3
       dist: precise
+  allow_failures:
+    - php: hhvm
 
 sudo: false