run build against PHP5.6 and hhvm but allow to fail

.travis.yml

@@ -4,7 +4,14 @@ php:
   - 5.4
   - 5.5
   - 5.6
-#  - hhvm # commented until composer or hhvm get fixed: https://github.com/facebook/hhvm/issues/1347
+  - hhvm
+
+# run build against PHP 5.6 and hhvm but allow them to fail
+# http://docs.travis-ci.com/user/build-configuration/#Rows-That-are-Allowed-To-Fail
+matrix:
+  allow_failures:
+    - php: hhvm
+    - php: 5.6
 
 services:
   - redis-server