TravisCI: Mute setup commands

We only care about the test and converage output for now
TravisCI: Added POSTGIS_VERSION environment variable
TravisCI: Only send notification mails on status change
TravisCI: Added PostGIS 2.0 build option

Travis should now test on both 1.5 and 2.0.3
README: Added TravisCI build status image
Merge pull request #20 from Turbo87/pg2.0

Added PostGIS 2.0 support to TravisCI