PostgreSQL: Don't specify a default PostgreSQL package

The user should specify which major version to use
(e.g. "services.postgresql.package = pkgs.postgresql92").  We can't
really provide a sensible default, because such a default would have
to be updated from time to time, and there is no automated upgrade
procedure.  So leave upgrading to the user.

modules/services/databases/postgresql.nix

@@ -53,7 +53,7 @@ in
       };
 
       package = mkOption {
-        default = pkgs.postgresql;
+        example = literalExample "pkgs.postgresql92";
         description = ''
           PostgreSQL package to use.
         '';