HHVM nightly is no longer supported on Ubuntu Precise

[dosten]

HHVM nightly is no longer supported on Ubuntu Precise. See travis-ci/travis-ci#3788 and facebook/hhvm#5220. We should remove hhvm-nightly from .travis.yml?

[stof]

I suggest running the testsuite on hhvm stable instead. We don't really need to rely on hhvm-nightly anymore here. hhvm would be fine too