Edx-ace

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

* BREAKING: Recipient objects now take `lms_user_id` instead of `username`
* New `braze_email` backend, needing the following new configuration:

* ACE_CHANNEL_BRAZE_API_KEY
* ACE_CHANNEL_BRAZE_APP_ID
* ACE_CHANNEL_BRAZE_REST_ENDPOINT (like `rest.iad-01.braze.com`)
* ACE_CHANNEL_BRAZE_CAMPAIGNS (an optional dictionary of ACE message names to Braze campaign identifiers)

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

* Updated he travis-badge in README.rst to point to travis-ci.com

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

* Use IntEnum to avoid silent failure in value comparisons

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

* Fixed Enum usage for Python 3.8 to avoid TypeError when comparing values

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

* Added support for Python 3.8
* Removed support for Django 2.0 and 2.1

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

* Fix trivial warning from deprecated use of attr library.