00bd23a Map shulker bullets to wither skulls in 1.9->1.8 (#731)

The previous mapping didn't really work, nothing would spawn.

I chose wither skull for a few reasons:

• They're also a hurting projectile
• They're fairly easy to hit
• Wind Charges are mapped to shulker bullets by ViaBackwards, and having them be wither skulls is quite convenient as they share the same motion properties (namely 0.95 air drag). I tried setting the invulnerable/aura entity data on the skull, though while that looks closer to both shulker bullets and wind charges, the air drag is much lower so movement gets desynced.