Fixed wrong use of vault api for perworldpermissions

A String with value null should be given to vault, when the group has to
be modified globally.
using e.g."playerRemoveGroup(player, groupnow);" causes inconsistens
behaviours for globale group change. The documentation of vault say's, it will use the current world to modify the group in.
Tested with PermissionsEx.

For further information, please read the javadoc of vault:
https://github.com/MilkBowl/Vault/blob/master/src/net/milkbowl/vault/permission/Permission.java#L490
https://github.com/MilkBowl/Vault/blob/master/src/net/milkbowl/vault/permission/Permission.java#L519