Add HHVM to Travis

This is a work in progress -- need to have this here to test Travis.

Until HHVM 2.5 is released and added to Travis, the builds will be failing because MySQLi hasn't been implemented yet. See http://www.hhvm.com/blog/3689/implementing-mysqli
I can confirm that all tests except 1 pass on HHVM nightly.

---
Tests skipped right now due to lack of HHVM support:
- runkit related tests
- gmp related tests
- selenium? (not sure -- need to see if Travis' HHVM works)