postgresql: Make 9.2 the default

PostgreSQL 8.3 is end-of-life so it shouldn't be our default anymore.

The problem with changing the default PostgreSQL is that it breaks
NixOS installations that have PostgreSQL enabled without specifying an
explicit PostgreSQL version, because PostgreSQL does not do automatic
schema migration if the major version changes.

Thus, it's always a good idea to specify the desired major version
explicitly:

  services.postgresql.package = pkgs.postgresql92;

(In fact, maybe we should remove the default value for
services.postgresql.package.)