Dropping support for 1.5 is up to you, but I agree it's probably not a good idea. I didn't drop support intentionally here, just wasn't testing against 1.5. Maybe a tox setup would be a good addition to make it easier to run tests on all supported configurations?

I think the best solution is probably just to make this conditional on Django version. That's a bit ugly but AFAIK there is no good way to set form class for choices in earlier versions. I can submit a PR for the conditional if you need (the relevant test should also be made conditional on Django version).