added libboost ppa to hhvm install recipe

Won't install, otherwise. Instructions taken from
https://github.com/facebook/hhvm/wiki/Prebuilt-Packages-on-Ubuntu-12.04
fix FC002: Avoid string interpolation where not required
fix FC041: Execute resource used to run curl or wget commands
fix FC013: Use file_cache_path rather than hard-coding tmp paths
using apt_repository instead of custom stuff
Merge pull request #344 from rwos/fix/hhvm

added libboost ppa to hhvm install recipe