Update TravisCI script

The new TravisCI build environment now ships with PostGIS 2.1 by default. I don't know if it is possible to install PostGIS 1.5 instead for testing, but in any case the config file needs to be adjusted to make it work with at least PostGIS 2.1.

(see http://about.travis-ci.org/blog/2013-11-18-upcoming-build-environment-updates/)