Merge pull request #3828 from hkdobrev/hhvm-nightly

Test on HHVM nightly releases. Allow to fail.

.travis.yml

@@ -18,6 +18,11 @@ php:
   - 5.6
   - 7.0
   - hhvm
+  - hhvm-nightly
+
+matrix:
+  allow_failures:
+    - php: hhvm-nightly
 
 before_script:
     - rm -f ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini