postgis: get rid of composableDerivation

In this change composableDerivation is removed in favor of
stdenv.mkDerivation.

nixos/modules/services/databases/postgresql.nix

@@ -122,7 +122,7 @@ in
       extraPlugins = mkOption {
         type = types.listOf types.path;
         default = [];
-        example = literalExample "[ (pkgs.postgis.override { postgresql = pkgs.postgresql94; }).v_2_1_4 ]";
+        example = literalExample "[ (pkgs.postgis.override { postgresql = pkgs.postgresql94; }) ]";
         description = ''
           When this list contains elements a new store path is created.
           PostgreSQL and the elements are symlinked into it. Then pg_config,

nixos/tests/postgis.nix

@@ -12,7 +12,7 @@ import ./make-test.nix ({ pkgs, ...} : {
         services.postgresql = let mypg = pkgs.postgresql100; in {
             enable = true;
             package = mypg;
-            extraPlugins = [ (pkgs.postgis.override { postgresql = mypg; }).v_2_4_0 ];
+            extraPlugins = [ (pkgs.postgis.override { postgresql = mypg; }) ];
         };
       };
   };