Switch to `hhvm` if hhvm is not `nightly`
Switch to `hhvm` if hhvm is not `nightly`

This resolves https://github.com/travis-ci/travis-ci/issues/6672

Versioned HHVM package needs `hhvm` for `phpenv global`.

craigcitro/travis-build,kevmoo/travis-build,JuliaCI/travis-build,kevmoo/travis-build,andyli/travis-build,andyli/travis-build,JuliaCI/travis-build,craigcitro/travis-build