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* Improved performance of `clean_orphan_obj_perms` management command
* Use bumpversion for versioning.
* Enable Python 3.6 testing
* Python 2.7, 3.4, 3.5, 3.6 are only supported Python versions
* Django 1.8, 1.10, and 1.11 are only supported Django versions
* Added explicity on_delete to all ForeignKeys