Prepare bindings to work with new RoboVM packaging mechanism

This fixes a compilation error in the Google Play Games Services bindings and adds a simple directive to the root build.gradle file. It will ensure that the robovm.xml and all native libraries in a project's libs/ folder are packaged into META-INF/robovm/ios/, as per https://github.com/robovm/robovm/issues/132

Once we publish these to Maven Central and/or SonaType, people can simply take the jar files and add them to their projects, either manually or via a dependency in their Gradle build scripts.