Fix Travis PostGIS/GDAL dependency problems.

Travis now currently uses the pgdg and ubuntugis-stable APT
repositories, which conflict in that e.g. pgdg contains
postgresql-9.1-postgis-2.1 (and PostGIS 2.1 is preinstalled before we
start) dependent on GDAL 1.9, but ubuntugis-stable's python-gdal is
dependent on GDAL 1.10.

Thanks to Mark Longair for investigating this.