Include nightly HHVM builds into the build matrix

.travis.yml

@@ -5,14 +5,16 @@ php:
   - 5.5
   - 5.6
   - hhvm
+  - hhvm-nightly
 
 # 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:
   fast_finish: true
   allow_failures:
-    - php: hhvm
     - php: 5.6
+    - php: hhvm
+    - php: hhvm-nightly
 
 services:
   - redis-server