Added hhvm to allowed failures
Added hhvm to allowed failures

As much as I hate to do this it’s been too unstable which is blocking
new pull requests from being processed in a timely way.

andrerom/Stash,alawnchen/Stash,andrerom/Stash,tedious/Stash,bankiru/Stash,csburton/Stash,AndrewCarterUK/Stash,jaredtking/Stash,tedious/Stash,graphis/Stash,theUniC/Stash,bankiru/Stash,localheinz/Stash,localheinz/Stash,jaredtking/Stash,theUniC/Stash,AndrewCarterUK/Stash,csburton/Stash,alawnchen/Stash,graphis/Stash