try to use postg(res|is)-9.3 instead of 9.1
CDB-3079 simplify installation relying on default postg(res|is)
CDB-3079 uses addons as per http://docs.travis-ci.com/user/using-postgresql/
CDB-3079 installs gdal
CDB-3079 tries to use cartodb flavour of postgres/postgis
CDB-3079 yet another try to use cartodb flavour of postgres/postgis
CDB-3079 enforces postgres user in preparation script
CDB-3079 enforces postgres user in preparation script
Merge branch 'CDB-3079' of https://github.com/CartoDB/CartoDB-SQL-API into CDB-2038