LookupError: App 'promotions' doesn't have a 'KeywordPromotion' model when upgrading to django oscar 1.6
In my case I didn't have a models.py in my overridden promotions app...  It just needs to contain this:

from oscar.apps.promotions.models import *
I have a django oscar shop that I'm attempting to upgrade to Oscar 1.6.  My shop extends Oscar's promotions model (although at this stage, does little with it).
It works fine if I remove the promotions app from the project, and it works fine in django 1.5.  However when I try to start the app in Oscar 1.6, I get the following error.
LookupError: App 'promotions' doesn't have a 'KeywordPromotion' model.
It seems that none of Oscar's promotions models are being loaded at startup.  There's a message in the release notes that says:

The majority of the Oscar class imports now use dynamic loading,
  instead of direct imports, for better extensibility and
  customisability.

Does anyone know if this, or otherwise is preventing the promotions model from loading?