applicationserver::hhvm: add boost backports ppa for beta

Per <https://github.com/facebook/hhvm/wiki/Prebuilt-Packages-on-Ubuntu-12.04>,
running HHVM on Precise now requires backports of boost.

Change-Id: Iea6cdceb90abdddc3a694b9eb45e220e600e1db7
Merge "applicationserver::hhvm: add boost backports ppa for beta" into production