Added testing support for 5.6 and hhvm-nightly

We like it when people upgrade to the latest version of php so we're supporting 5.6 out of the gate. 

Testing against hhvm-nightly builds will help us identity any problems that need to be reported back to the HHVM project, while also keeping compatibility issues in mind. Since it's against a dev version we're not enforcing that the tests pass (which we *are* doing with hhvm).