Enable PostGIS extension for PostgreSQL tests

This is more like a proposal. Since PostGIS is one of the most popular extensions of PostgreSQL, it might be a good idea to enable it on Travis and make sure DBAL works with it.

At the moment, DBAL's Schema Tool does not work with PostGIS because it cannot handle some VIEWS added by PostGIS.

See: https://travis-ci.org/jsor/dbal/jobs/40528527