Add git submodule update to hhvm build

The upstream repo changed to use a submodule for third-party
components on 2014-04-12 [0].

[0]: https://github.com/facebook/hhvm/commit/4c3be41811bea45f2f3be55541c05b817806738b

Change-Id: I60b3c931a5eb5d1359393490b8b4536a45021696