Upgrade to latest Django Axes

[loicteixeira]

Previously, the package did not define a range for its django-axes dependency. Since 2.3.3 is the last compatible version with this package, bea4bf6a1122064be2d49c9693d56ecfe3549b91 adds the corresponding range limit.

django-axes version 3.0 comes with some improvements for Django 1.11 (FWIW, there is even a 4.0 now). However it drops compatibility for Django prior to 1.11 which would in turn will force us to drop support for Wagtail prior to 1.10. In addition, it has many breaking changes (as you can see with jazzband/django-axes#240).

In any case, I think it's important to upgrade to the latest of django-axes since it's a critical dependency for this package but it might be important to write tests first (see #6) to make sure that the upgrade doesn't introduce more issues.