Remove HHVM testing to fix failing build caused by HHVM bug.
Remove HHVM testing to fix failing build caused by HHVM bug.

The bug was fixed in https://github.com/facebook/hhvm/commit/b786320582e086630bbeb65043c3be5680257ba0,
now it needs to hit a stable release, then that release needs to be deployed on Travis.

commerceguys/zone