[tests] Remove hhvm-nightly from Travis CI configuration.

See https://github.com/travis-ci/travis-ci/issues/3788 for details.

.travis.yml

@@ -6,7 +6,6 @@ php:
   - 5.6
   - 7.0
   - hhvm
-  - hhvm-nightly
 branches:
   except:
     - v0.5
@@ -22,5 +21,4 @@ script:
 matrix:
   allow_failures:
     - php: hhvm
-    - php: hhvm-nightly
   fast_finish: true