As in `test_model.py` -> `test_models.py`

The idea of splitting integration from unittests is good in theory
but leads to a lot of mental overhead. Besides whenever a tests
interacts with a database it isn't a unittest anyway.

Better to use filenames which are commonly used in the python word then
custom ones.

Previously, weight based shipping methods accepted an upper charge
field. When no weight band matched the baskets weight, this was
returned. I am not aware of a single retailer who calculates shipping
charges like that. It's impossible to pick a sensible value for a
retailer, as there's no upper limit to the amount of items ordered, and
any shipping charges would have to cover costs for that.

Seems ValidationError got a rewrite for 1.6.

Based on http://stackoverflow.com/questions/12384460/allow-only-positive-decimal-numbers

Hmmm, Oscar has a PositiveInteger model field but I suspect we don't
really need it (or should refactor it to use validators).