Get Travis CI tests passing on all python versions

Add a dependency to `.travis.yml` to use the ubuntugis stable repo to install GEOS.  The default repo was installing GEOS 3.2.2, which doesn't have `polygonize_full`.  The ubuntugis stable repo currently has GEOS 3.3.8, which works fine.

One small fix was required for python 3.2 to explicitly cast a string as bytes.

[Tests on Travis CI on this branch](https://travis-ci.org/kjordahl/Shapely/builds/14105156) are now passing for 2.6, 2.7, 3.2 and 3.3.  If you enable the service hook for the main repository on your [Travis CI profile](https://travis-ci.org/profile) we'll get tests running for all commits and pull requests.