Set CraftTravelAgent.DEFAULT to a non-null value.

Currently, `CraftTravelAgent` will call `s()` on the passed-in `WorldServer` in order to set `DEFAULT`. However, `s()` will always return null at this point, because `WorldServer.P` will still be `null`, as it is set after the constructor is called. Instead, `CraftTravelAgent.DEFAULT` should be set to the instance that is being constructed.

Leaky: https://bukkit.atlassian.net/browse/BUKKIT-3578