[wip] Install a newer version of nginx

This will use the Launchpad PPA to install the latest stable build of nginx.

I'm not sure if this is something that we should submit upstream to the nginx role or not - it'll break on Debian systems (non-Ubuntu) and newer Ubuntu images would need an additional `python-software-properties` package. I didn't add them in here because the images *we're* using already have `add-apt-repository` installed... and the package that provides it changed names.