Update django-test-plus to 1.0.15

[pyup-bot]

There's a new version of django-test-plus available.
You are currently using 1.0.11. I have updated it to 1.0.15

Changelog

1.0.15

---

• Added helper methods for more HTTP methods like put, patch, and trace
• Added assertResponseContains and assertResponseNotContains

1.0.14

---

• Fixed documentation typo
• Added response_400() test
• Added Guinslym and David Arcos to AUTHORS.txt

1.0.13

---

• Added response_401() test
• Fixed situation where User models without a 'username' field could not be
  used as easily. Now credential field is automatically determined.
• Fixed assertLoginRequired when settings.LOGIN_URL is a named URL pattern
• Removed support for Django 1.4.x as it is well beyond it's end of life and causes a headache for supporting newer releases

1.0.12

---

• Fixed incorrect documentation
• Added response_405 and response_410 test methods

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

Happy merging! 🤖

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #131 into master will not change coverage

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  Methods         0      0          
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  Hits           96     96          
  Misses          0      0          
  Partials        0      0          

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