This also adds the DEFAULT_AUTO_FIELD to the app's Django settings to
resolve this warning when running `migrate`:
```
ingredients.Category: (models.W042) Auto-created primary key used when not defining a primary key type, by default 'django.db.models.AutoField'.
HINT: Configure the DEFAULT_AUTO_FIELD setting or the IngredientsConfig.default_auto_field attribute to point to a subclass of AutoField, e.g. 'django.db.models.BigAutoField'.
```
* Update travis and tox
* Use xenial distribution
* Don't install coveralls twice
* Add black and flake8 tox commands
* Remove Python 3.5 test for Django master
* Fix indent
* Ignore migrations
* Remove black for now
* Run black formatting (#668)
* Run black format
* Update makefile
* Add black to travis build
