Took a bit longer than expected for testing and finding out the extent of compatibility.
Note on SkeletonGraphic:
UnityEngine.UI in 5.1 and below do not allow you to push your own meshes into the UI system, so it can't be used to render Spine skeletons.
I've disabled SkeletonGraphic with a compiler directive (while leaving the public fields for serialization) and left a warning for pre-5.2 users.
The fix should be up in a few minutes.
Thanks again for reporting.