Update CONTRIBUTING.rst

[JohnHBrock]

I updated the contribution guidelines, but I had to guess at some of the details:

• I replaced the travis URL with the github CI URL, since the travis URL seems to be dead, but maybe it just moved?
• I left the mention of tox in there, but maybe it's not being used anymore?
• I changed the mention of supported python versions to match what pypi says, but someone should verify.

Also, I removed the dollar signs before the shell commands since that lets you copy and paste multiple lines into the shell at once without first needing to modify them.

[terricain]

Looks good, thanks. Yeah I've not touched that doc in ages. Tox isn't used anymore.