Update django-debug-toolbar to 2.1

[pyup-bot]

This PR updates django-debug-toolbar from 2.0 to 2.1.

Changelog

2.1

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

* Changed the Travis CI matrix to run style checks first.
* Exposed the ``djdt.init`` function too.
* Small improvements to the code to take advantage of newer Django APIs
and avoid warnings because of deprecated code.
* Verified compatibility with the upcoming Django 3.0 (at the time of
writing).

Links

• PyPI: https://pypi.org/project/django-debug-toolbar
• Changelog: https://pyup.io/changelogs/django-debug-toolbar/
• Repo: https://github.com/jazzband/django-debug-toolbar