Revert "Disable hhvm until I figure out why it doesn't work"

https://github.com/facebook/hhvm/pull/2965 is now resolved.

This reverts commit 0df2a3984d97de58872daf210b4271bc7cf57c29.

Change-Id: Ideec90505b1ce35b209439a9e90d8e02e87a4f1a