Update django to 2.1.7

[pyup-bot]

This PR updates Django from 2.1.5 to 2.1.7.

Changelog

2.1.7

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

*February 11, 2019*

Django 2.1.7 fixes a packaging error in 2.1.6.

Bugfixes
========

* Corrected packaging error from 2.1.6 (:ticket:`30175`).


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

2.1.6

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

*February 11, 2019*

Django 2.1.6 fixes a security issue and a bug in 2.1.5.

CVE-2019-6975: Memory exhaustion in ``django.utils.numberformat.format()``
--------------------------------------------------------------------------

If ``django.utils.numberformat.format()`` -- used by ``contrib.admin`` as well
as the the ``floatformat``, ``filesizeformat``, and ``intcomma`` templates
filters -- received a ``Decimal`` with a large number of digits or a large
exponent, it could lead to significant memory usage due to a call to
``'{:f}'.format()``.

To avoid this, decimals with more than 200 digits are now formatted using
scientific notation.

Bugfixes
========

* Made the ``obj`` argument of ``InlineModelAdmin.has_add_permission()``
optional to restore backwards compatibility with third-party code that
doesn't provide it (:ticket:`30097`).


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

Links

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

[pyup-bot]

Closing this in favor of #331