[PyUP] Update readme-renderer to 18.1

[pyup-bot]

This PR updates readme-renderer from 17.2 to 18.1.

Changelog

18.1

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* Change Github-flavored Markdown identifier from ``gfm`` to ``GFM``.

18.0

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* Add support for GitHub-flavored Markdown. (67)
* Switch to cmarkgfm completely for rendering Markdown. (68)
* Warn about missing long description. (69)
* Make Github-Flavored Markdown the default variant (73)

17.4

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* All renderers now accept arbitrary ``kwargs`` for future-proofing.

17.3

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* Gracefully handle new exceptions from bleach.

Links

• PyPI: https://pypi.python.org/pypi/readme-renderer
• Changelog: https://pyup.io/changelogs/readme-renderer/
• Repo: https://github.com/pypa/readme_renderer

[pyup-bot]

Closing this in favor of #66