Allow teleportation of entities on vehicles. Fixes BUKKIT-4210

Up until Minecraft version 1.5 it was not possible to teleport entities
within vehicles. With the 1.5 update came the change in the Minecraft
teleportation logic to dismount before teleporting the entity, if
applicable.

This commit ammends the existing CraftBukkit logic for rejecting
teleportation for entities in vehicles to permit the action. Due to this
change, CraftBukkit is now in-line with Minecraft 1.5 teleportation logic.