Bug#930495: Recommends should be Depends

[Michael Reed]

Package: postgis
Version: 2.5.1+dfsg-1

The PostGIS extension for postgresql has apparently been moved out of
the PostGIS package.  One must now install postgresql-11-postgis-2.5
and postgresql-11-postgis-2.5-scripts to get the needed functionality.

These packages are only Recommends though, so someone upgrading from
stretch with postgis package to buster will not actually get the
postgis extension, causing the pg_upgradecluster to fail spectacularly.

It seems like postgis should depend on both postgresql-11-postgis-2.5
and postgresql-11-postgis-2.5-scripts.