added add-apt-repository ppa:mapnik/boost

Hi

First at all thanks for this excellent project, i was playing with it and currenlty looks like not works.

I checked the [documentation](https://github.com/facebook/hhvm/wiki/Prebuilt-Packages-on-Ubuntu-12.04) of HHVM and looks like is missing the command `add-apt-repository ppa:mapnik/boost`

With this patch now works perfect!

Regards