fixed #1 (GWT)

Looks like someone copied my build script ;) You actually discovered a mistake. The `archiveSources` and `jar` tasks did not include the GWT module file, so the modules could not be found when you tried to inherit them. Here's a modified version of my new build script.
P.S.: One cannot use my libgdx-utils with yours in one project for GWT because the modules have the same name.