TravicCI: Install coverage dependency in after_script section

We don't need this to run the tests and the results will be visible a
bit faster now.
TravisCI: Moved psql -U to the front
TravisCI: Added comments
TravisCI: Enabled raster support
TravisCI: Install PostGIS 2.x via CREATE EXTENSION
Merge pull request #26 from Turbo87/travis

TravisCI: Use "CREATE EXTENSION" to install PostGIS 2.x