fix wrong method called in SlicePrivilege.save
diff --git a/xos/core/models/slice.py b/xos/core/models/slice.py
index 0c55791..4fc8489 100644
--- a/xos/core/models/slice.py
+++ b/xos/core/models/slice.py
@@ -134,7 +134,7 @@
     def save(self, *args, **kwds):
         if not self.user.is_active:
             raise PermissionDenied, "Cannot modify role(s) of a disabled user"
-        super(SlicePrivilege, self).delete(*args, **kwds)    
+        super(SlicePrivilege, self).save(*args, **kwds)
 
     def can_update(self, user):
         return user.can_update_slice(self.slice)