don't use attname in the ORM update kwargs, not recognised

I had a denormalised field like:

```python
@denormalized(models.ForeignKey, to='Team', null=True, blank=True)
@depend_on_related('Team', foreign_key='member')
def team(self):
    i, team = self._team_and_member_index()
    return team.pk
```

When I tried to `./manage.py denorm_rebuild` (or do a `rebuildall()` in python console) I was getting the error:

    FieldDoesNotExist: Member has no field named 'team_id'

I think it's wrong to try and use the `attname` in the update query... this fixes it for me and I'm pretty sure shouldn't break anything else.