Wagtail-cache is incompatible with django-developer-panel

I think this is an issue at the [django-develeoper-panel](https://github.com/loftylabs/django-developer-panel) end though so if I get time I'll do some testing and open an issue or pull request there. The problem is [this line](https://github.com/loftylabs/django-developer-panel/blob/52fd2666a158b197fdd643d3c2c4d9bbc5ba230a/djdev_panel/middleware.py#L175) in the middleware, our response has no `context_data` attribute so this line throws an exception. It's easy enough to make that return if the attribute is missing, but there may be more to it than that.

Might be worth adding to known issues in the readme for now though.