Fixed the installation of HHVM nightly on Travis

This is an attempt to fix the installation of HHVM nightly on Travis, which is failing because of dependency versions: https://travis-ci.org/doctrine/dbal/jobs/23685468

Once Travis updates its VM, we will get HHVM 3.0.1, but the process of deploying the new versions seems staled: https://github.com/travis-ci/travis-ci/issues/2126