Fix: Remove hhvm-nightly from build matrix

This PR

* [x] removes `hhvm-nightly` from the build matrix 

> HHVM nightly is no longer supported on Ubuntu Precise. See travis-ci/travis-ci#3788 and facebook/hhvm#5220

.travis.yml

@@ -15,12 +15,9 @@ php:
   - 5.6
   - 7.0
   - hhvm
-  - hhvm-nightly
 
 matrix:
   fast_finish: true
-  allow_failures:
-    - php: hhvm-nightly
 
 before_script:
   - rm -f ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini