Updated offer.custom module to load models using get_model()
This change ensures that the module can be loaded if models are customized in forked apps.
Fixes
from django.core import exceptions
from django.db import IntegrityError
-from oscar.apps.offer.models import Benefit, Condition
from oscar.core.loading import get_model
+Benefit = get_model('offer', 'Benefit')
+Condition = get_model('offer', 'Condition')
Range = get_model('offer', 'Range')
