Fix wheezy

We were previously relying on a VM whose setup I can't replicate, it had some
random gcc pulled from some ubuntu repo whose lineage I can't find. Instead,
I rebuilt the VM with a few packages pulled from jessie. This required some repo
munging on our end, but this is the update to the packaging script.

https://github.com/facebook/hhvm/wiki/Building-and-installing-HHVM-on-Debian-7
and
https://github.com/facebook/hhvm/wiki/Prebuilt-Packages-on-Debian-7
should have the relevant info to reproduce what I've done.