Post-Django 1.11 Upgrade Cleanup

[schbetsy]

• Update setup.py and tox.ini to reflect that we don't support Django 1.8
• Update links to Django documentation to point to current version
• Allow flexible version of djangorestframework and django-haystack

Checklist

• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the front end playbook
• Passes all existing automated tests
• New functions include new tests
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged
• Visually tested in supported browsers and devices
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[coveralls]

Coverage remained the same at 100.0% when pulling b95b899 on post-dj111-cleanup into fad0b63 on master.