Update django to 2.1.8

[pyup-bot]

This PR updates Django from 2.1.7 to 2.1.8.

Changelog

2.1.8

==========================

*April 1, 2019*

Django 2.1.8 fixes a bug in 2.1.7.

Bugfixes
========

* Prevented admin inlines for a ``ManyToManyField``\'s implicit through model
from being editable if the user only has the view permission
(:ticket:`30289`).


==========================

Links

• PyPI: https://pypi.org/project/django
• Changelog: https://pyup.io/changelogs/django/
• Homepage: https://www.djangoproject.com/

[coveralls]

Pull Request Test Coverage Report for Build 811

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.102%

---

Totals
Change from base Build 807:	0.0%
Covered Lines:	1794
Relevant Lines:	2421

---

💛 - Coveralls

[pyup-bot]

Closing this in favor of #184