Add coverage reports to py.test runner

Summary: Of note this now forces the pytest-cov plugin to be installed unless they turn it off.

Test Plan:
```
../arcanist/bin/arc unit

COVERAGE REPORT
     97%     changes/models/test.py
    100%     tests/changes/api/serializer/models/test_testcase.py
    100%     changes/api/serializer/models/testcase.py
 UNIT OKAY  No unit test failures.
Updated an existing Differential revision:
        Revision URI: https://tails.corp.dropbox.com/D43387

Included changes:
  M       changes/api/serializer/models/testcase.py
  M       changes/models/test.py
  M       tests/changes/api/serializer/models/test_testcase.py
```

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8667