Whitelist the coverage command in tox.ini to avoid some warnings.

[aerostitch]

This change gets rid of the following warning message in the travis builds for py{36}-django{111,20,-master} and py{35,36,37}-django21:

py37-django21 runtests: commands[0] | coverage run pylint_django/tests/test_func.py -v
WARNING: test command found but not installed in testenv
  cmd: /home/travis/virtualenv/python3.7-dev/bin/coverage
  env: /home/travis/build/PyCQA/pylint-django/.tox/py37-django21

It's just for cleanup purposes.

Thanks,
Joseph

[coveralls]

Pull Request Test Coverage Report for Build 556

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.697%

---

Totals
Change from base Build 550:	0.0%
Covered Lines:	613
Relevant Lines:	699

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 556

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.697%

---

Totals
Change from base Build 550:	0.0%
Covered Lines:	613
Relevant Lines:	699

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 556

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.697%

---

Totals
Change from base Build 550:	0.0%
Covered Lines:	613
Relevant Lines:	699

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 556

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.697%

---

Totals
Change from base Build 550:	0.0%
Covered Lines:	613
Relevant Lines:	699

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 561

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.697%

---

Totals
Change from base Build 559:	0.0%
Covered Lines:	613
Relevant Lines:	699

---

💛 - Coveralls

[atodorov]

@aerostitch can you rebase so I can merge cleanly.

I tried resolving the conflicts via GitHub's UI and it made another commit which I don't like.

[aerostitch]

Thanks, I'll have a look asap.

[aerostitch]

PR updated @atodorov . Note that if you have additional merge commits you can use the "Squash and merge" option when merging that squashes the commits together and adds the PR number in parenthesis at the end if the commit message (which... me likey! 😆 ).