Remove the GOV.UK PPA from our Puppet

1. I've added some packages that I know we need to [our PPA](https://launchpad.net/~gds/+archive/performance-platform) - if you know of any I've missed, please let me know.
2. I'm not sure if this will remove the PPA from our machines, or if we need to purge it to do that. I'm also not sure it matters either way.
3. This will upgrade Nginx from 1.4.1 to 1.4.4, which fixes [a security advisory](http://mailman.nginx.org/pipermail/nginx-announce/2013/000125.html). Again, this may only affect new machines. As far as I can tell we don't have a clean way to ensure a version of a class is used.