Update pytest-runner to 4.2

[pyup-bot]

This PR updates pytest-runner from 4.0 to 4.2.

Changelog

4.2

===

* 40: Remove declared dependency and instead assert it at
run time.

4.1

===

* 40: Declare dependency on Setuptools in package metadata.

Links

• PyPI: https://pypi.python.org/pypi/pytest-runner
• Changelog: https://pyup.io/changelogs/pytest-runner/
• Repo: https://github.com/pytest-dev/pytest-runner

[pyup-bot]

Closing this in favor of #128