[Security] Bump django from 1.11.15 to 1.11.20 in /arches/install

[dependabot-preview]

Bumps django from 1.11.15 to 1.11.20. This update includes security fixes.

Vulnerabilities fixed

Sourced from The GitHub Security Advisory Database.

Low severity vulnerability that affects django
In Django 1.11.x before 1.11.18, 2.0.x before 2.0.10, and 2.1.x before 2.1.5, an Improper Neutralization of Special Elements in Output Used by a Downstream Component issue exists in django.views.defaults.page_not_found(), leading to content spoofing (in a 404 error page) if a user fails to recognize that a crafted URL has malicious content.

Affected versions: < 1.11.18

Sourced from The GitHub Security Advisory Database.

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().

Affected versions: >= 1.11.0, < 1.11.19; >= 2.0.0, < 2.0.11; >= 2.1.0, < 2.1.6

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[coveralls]

Coverage remained the same at ?% when pulling 1e6e49e on dependabot/pip/arches/install/django-1.11.20 into 8b91649 on master.

[dependabot-preview]

Superseded by #4929.