Update requirements in readme and tests

[mikemanger]

A few changes to make it clearer what versions are now supported:

• Updates README.rst and setup.py to indicate what versions of Python and Django are supported (fixes #159)
• Brings tox.ini inline with the workflow file
• Remove deprecated testing requirements from setup.py

[mikemanger]

Is travis still used? I can try and update that too or might be good to remove the config file if not used anymore.

[blag]

I don't think Travis is still used, no. Removing that would be nice.

[mikemanger]

Thanks, I have removed Travis and replaced the badge in the readme file.

[goinnn]

Thanks @mikemanger. Add these chaneges in next commit: [85a608c0e2200959f7a4dfc73f8d96427d5d7f4b]