Update django-test-plus to 1.0.18

[pyup-bot]

There's a new version of django-test-plus available.
You are currently using 1.0.15. I have updated it to 1.0.18

These links might come in handy: PyPI | Changelog | Repo

Changelog

1.0.18

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• Allow custom Request objects in get() and post()
• Begin testing against Python 3.6 and Django 1.11

1.0.17

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• Added assertResponseHeaders

1.0.16

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• Added print_form_errors utility

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖

[codecov-io]

Codecov Report

Merging #261 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #261   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines          64     64           
=====================================
  Hits           64     64

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