Update django to 1.11.18

[pyup-bot]

This PR updates Django from 1.11.17 to 1.11.18.

Changelog

1.11.18

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

*January 4, 2019*

Django 1.11.18 fixes a security issue in 1.11.17.

CVE-2019-3498: Content spoofing possibility in the default 404 page
-------------------------------------------------------------------

An attacker could craft a malicious URL that could make spoofed content appear
on the default page generated by the ``django.views.defaults.page_not_found()``
view.

The URL path is no longer displayed in the default 404 template and the
``request_path`` context variable is now quoted to fix the issue for custom
templates that use the path.


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Links

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