Test on HHVM nightly releases. Allow to fail.

Nightly releases could change in unpredictable way.
So they should not fail the whole build.

However we could catch problems and breaking changes
on next versions of HHVM early.

.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