Update HHVM OSS third-party commit to latest

Summary: Right now `.git/modules/third-party/HEAD` is pointing to `f15159847e941c874f5a21386df0603f22a08b98`. We need to update this to the most recent commit `fa8d24dcbed7ddd6bfad9cf89cc691b12830ffb4` so that when we `git clone` the open source code, we get things like `fastlz` which have been added since `f1515984`

Test Plan: Code using stuff like `fastlz` will compile (e.g. D1382905) in oss-land

Reviewers: sgolemon, fredemmott, liranuna, ptarjan

Reviewed By: ptarjan

Subscribers:

Differential Revision: https://phabricator.fb.com/D1384497

Tasks: