Update webargs to 5.1.1

[pyup-bot]

This PR updates webargs from 5.1.0 to 5.1.1.

Changelog

5.1.1

******************

Bug fixes:

* Fix installing ``simplejson`` on Python 2 by
distributing a Python 2-only wheel (:issue:`363`).

Links

• PyPI: https://pypi.org/project/webargs
• Changelog: https://pyup.io/changelogs/webargs/
• Repo: https://github.com/marshmallow-code/webargs

[coveralls]

Coverage remained the same at 86.888% when pulling 81444bb on pyup-update-webargs-5.1.0-to-5.1.1 into 786e7c8 on master.