Enable 1.6.3 Snapshot support by default

1.6.3 is coming out soon, so I figured getting this patch ready beforehand was a good idea.

1.6.3 doesn't have any packet changes ([Source](http://wiki.vg/Pre-release_protocol)), so I think the reason the protocol was bumped was due to the authentication bug. That's already included in the 1.6.2 CraftBukkit, so no need to port that over. This is pretty much just the patch from a while ago for the 1.6.1 -> 1.6.2, but now for 1.6.2 -> 1.6.3.