Edx-completion

------------------------------------------------
* Support tracking completion of XBlocks in any "learning context", such as in
a content library, and not just in courses. To keep the code clean, this has
been done as a **breaking change** to the python API. (The API has been
simplified so that it's generally only necessary to pass in a block key /
usage key rather than block key + course key.) The REST API is unchanged.

------------------------------------------------
* Updated credentials for PyPI deployment via token.

------------------------------------------------
* Switch blocks_to_mark_complete_on_view() to return a list of XBlocks instead of a set. Many XBlocks aren't hashable;
the old implementation allowed subtle bugs under Python 2.7 but triggers an immediate error under 3.5.

------------------------------------------------
* Unpin django-rest-framework requirements. This is a potentially **breaking change** if people were
relying on this package to ensure the correct version of djangorestframework was being installed.
* Remove the AUTHORS file and references to it.

------------------------------------------------

* Fix the 403 error occurring for completion-batch API for requests coming from the iOS devices

------------------------------------------------
* Updated edx-drf-extensions imports. Completion will no longer work with
outdated versions of edx-drf-extensions.