Your code worked fine after I turned off hardware skinning like so:
SkeletonControl skeletonControl = new SkeletonControl(skeleton);
skeletonControl.setHardwareSkinningPreferred(false); // added code
model.addControl(skeletonControl);
In 3.0, software skinning was the default, but not in 3.1. That is one of the known gotchas in the 3.0->3.1 transition. For other porting tips, see the forum topic: