spigot 1.13.2+ support.

[BlitzOffline]

The plugin uses a few methods and events that didn't exist in spigot 1.13.2. I'll make a list of them here and you guys can just check the boxes when they're fixed.

• Material#isAir https://paste.helpch.at/poxocabuga.cs) - Fixed in #84
• PlayerHideEntityEvent (https://paste.helpch.at/ohaforoloy.coffeescript) - Fixed in #84

Won't close this as if there's future related problems we'll just add them here.

[Kqliber]

these commits should be reverted as we're only supporting 1.16.5+ now 😌