Feature/use postgis extension

This PR switches ubuntu platforms over to the [ubuntugis stable ppa](https://launchpad.net/~ubuntugis/+archive/ubuntu/ppa)

Also, with postgresql >= 9.1 we can [skip reading sql files](https://docs.djangoproject.com/en/dev/ref/contrib/gis/install/postgis/#creating-a-spatial-database-with-postgis-2-0-and-postgresql-9-1 ) and use the same postgis "create extension" on both ubuntu 12.04 and 14.04

The plan is to refine/add support for debian in future pull requests.  This is not a feature branch, but I think it should help with that future debian goal. I'm new to chef, so any reviews and feedback is welcomed.