This line might introduce an intermittent bug if the BigIntegerField is in fact an IntegerField and mommy generates a big integer.

On the begining of `mommy.py` we have:

```
try:
    from django.db.models import BigIntegerField
except ImportError:
    BigIntegerField = IntegerField
```

It don't seem to be a problem having only a gen_integer generator doing all the integer related work. What do you think?