Fixes for postgis  PointField and MultiPolygonField

Hi,

I created a PR for the problems I was having when using sqldiff on PostGIS. The problem was even if I already had the schemas in my DB, sqldiff was showing me to drop and add them again. You can see them below.

```
BEGIN;
-- Application: destinations
-- Model: Country
DROP INDEX "destinations_country_geometry";
-- Comment: Unknown database type for field 'geometry' (16946)
ALTER TABLE "destinations_country"
	ALTER "geometry" TYPE geography(MULTIPOLYGON,4326);
-- Model: Destination
DROP INDEX "destinations_destination_point";
-- Comment: Unknown database type for field 'point' (16392)
ALTER TABLE "destinations_destination"
	ALTER "point" TYPE geometry(POINT,4326);
-- Model: DestinationPhoto
DROP INDEX "destinations_destinationphoto_point";
-- Comment: Unknown database type for field 'point' (16392)
ALTER TABLE "destinations_destinationphoto"
	ALTER "point" TYPE geometry(POINT,4326);
COMMIT;
```

After the fix:

``-- No differences``