Don't run our tests on hhvm-nightly anymore.

Hhvm does not build hhvm-nightly packages anymore for ubuntu-precise. (travis-ci/travis-ci#3788)
This means that this feature is broken and won't work anymore in the (near) future.

.travis.yml

@@ -8,11 +8,9 @@ matrix:
         - php: 5.6
         - php: 7
         - php: hhvm
-        - php: hhvm-nightly
     allow_failures:
         - php: 7
         - php: hhvm
-        - php: hhvm-nightly
 
 script:
     - phpunit -c app/ --coverage-clover=coverage.clover