Fix Travis build

A few fixes:

- Removed UbuntuGIS-unstable since Travis now ships with PostGIS and PostgreSQL

- The latest version of pip is more strict and since ModestMaps is not hosted
  by Pypi, it needs the --allow-external ModestMaps --allow-unverified ModestMaps flags

- The recommended way to setup databases in Travis is to use the before_script
  section.