Use the result of EntityPortalExitEvent. Fixes BUKKIT-3559.

Changing pretty much anything about EntityPortalExitEvent does not change the outcome of the event. The only exception is if the original to location or after velocity is mutated rather than being set to a new object, as the objects used to construct the event are the live ones passed in to PortalTravelAgent.adjustExit(Entity, Location, Vector). This is because the parameters passed in to adjustExit are assigned rather than mutated after the event is fired, and as Java passes by value this assignment has no effect.