add HHVM as an optional build target

I don't expect Mockery to pass tests in HHVM straight off, and I expect this is down to an incomplete Reflection implementation in HHVM.  However, testing against HHVM will hopefully provide useful information that could help expedite bringing HHVM to parity so tests using Mockery can be used in that environment.
Merge pull request #233 from shieldo/patch-1

add HHVM as an optional build target