goocanvasmm2.updateScript: change version policy

This package is dead and stuck on unstable version so odd-unstable policy tried to downgrade it, even though it was intentionally pinned on unstable because no stable version is available.

pkgs/development/libraries/goocanvasmm/default.nix

···
     updateScript = gnome.updateScript {
       packageName = pname;
       attrPath = "goocanvasmm2";
-      versionPolicy = "odd-unstable";
+      versionPolicy = "none"; # stable version has not been released yet, last update 2015
     };
   };