Make README compliant with PyPI RST renderer

[tortila]

Signed-off-by: Oliwia Zaremba oliwia.zaremba@zalando.de

Recent release failed because of the following error:

[UploadError]                                                    
HTTP Error 400: The description failed to render for 'text/x-rst'. See https  
://pypi.org/help/#description-content-type for more information.   

By installing readme-renderer and following its hints, I made small changes to README.rst so that it's compliant with renderer on PyPI. I also added this check in .travis.yaml (part of make lint now).

Post-fix to #57 (well, this is where I found the issue, not where I introduced it)

Not bumping the version because the most recent releases failed.

Types of Changes

• Configuration change

Review

Reviewers' checklist:

• If this PR implements new flows or changes existing ones, are there
  good tests for these flows?
  If this PR rather removes flows, are the obsolete tests removed as well?
• Is the documentation still up-to-date and exhaustive? This covers both
  technical (in source files) and functional (under docs/) documentation.
• Is the changelog updated?
• Does the new version number correspond to the actual changes from this PR?
  In doubt, refer to https://semver.org.

After this PR

Check if release is successful, check how README renders on PyPI.

[zincr]

🤖 zincr found 0 problems , 0 warnings

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[tortila]

FYI I thought it would be cool to have this supported by Poetry, so I created this feature request: python-poetry/poetry#1108