Remove Travis config and all its references

[browniebroke]

• Remove Travis config as it's no longer used
• Remove old README.rst (I believe it's no longer needed)
• Replace Travis badge by GitHub action in the docs

[browniebroke]

@alanjds in case you missed this PR, just a small clean up.

[alanjds]

@browniebroke thanks for the cleanup. Yeah, the README.rst is no longer needed, as PyPI can render Markdown nowdays.