Update coveralls to 1.8.1

[pyup-bot]

This PR updates coveralls from 1.7.0 to 1.8.1.

Changelog

1.8.1

Bug Fixes

* **dependencies:**  pin `coverage` to `< 5.0`, since the current `5.0` alphas are
                  introducing breaking changes. Once `5.0` is stable, we'll
                  remove the pin.

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1.8.0

Features

* **flag:**  allow disabling SSL verification ([2e3b5c61](2e3b5c61))

Bug Fixes

* **git:**  fix support for case where git binary is missing ([5bbceaae](5bbceaae))

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Links

• PyPI: https://pypi.org/project/coveralls
• Changelog: https://pyup.io/changelogs/coveralls/
• Repo: http://github.com/coveralls-clients/coveralls-python