Fix Multiverse/Multiverse-NetherPortals#68

This was caused by Bukkit/CraftBukkit@de86f8c

When the portal in question is a Nether portal, re-enable useTravelAgent().

A build with this fix for testing can be found here:
http://dl.dropbox.com/u/71355/Multiverse-NetherPortals-2.5_issue68.jar