Original model run through meshlab and blender to fix some mesh issues. Use fixed2 unless you have a reason not to, info of difference below.

All credit for the model belongs to the original author I just clicked some buttons on this one to make some software happy :-)

Update:
I have uploaded a second version of the model(skull_and_crossbones_fixed2.stl) which slices off a small amount at the back to make it flat for an easy print. The original model has a small cross on the back and the bones are raises very slightly above the back of the skull. Shaving a fraction of a mm off the back makes printing much easier.