Update travis.yml to use ubuntugis gdal
Update travis.yml to use ubuntugis gdal

The .travis was configured to use the travis_install_dependencies script to install GDAL. 
We decided to only test against one version of GDAL for now, so the best GDALVERSION would be the one at ubuntugis:stable.
johntruckenbrodt/pyroSAR,johntruckenbrodt/pyroSAR