Travis CI setup

This PR includes complete and functional Travis CI service configuration for GEOS as requested here http://trac.osgeo.org/geos/ticket/657

It has been successfully tested against my fork the repo. Here is history of builds against ```mloskot/libgeos```: https://travis-ci.org/mloskot/libgeos

Note, I have already enabled Travis CI hook for ```libgeos/libgeos``` repository, so once this changeset is merged, everything should work automatically with history of builds at https://travis-ci.org/libgeos/libgeos