Add some sort of basic sanity testing for PRs

[jeffwidman]

As demonstrated in #11, we need some sort of basic sanity checking for PRs.

Ideally we test each PR against all versions of python it claims to support. That way, if there's a packaging issue in an older setuptools, pip, python etc we catch it.

I think all we need to do is wire up something that starts a cassandra docker image and verifies that cqlsh can connect to it. That would have flagged all the issues I've hit in this repo so far.

Ideally it would both install cqlsh as a module within another super simple python program and install it as an executable.

Test matrix should probably be supported Python versions (tox is probably handy) * OS (Linux/Windows). May also want to run against multiple Cassandra versions, although I tend to think that's the domain of the Cassandra project as here we really only control the packaging aspect.

But for starters, even getting one flow working would be great.

[bschoening]

It's fairly simple to setup Travis and its free for open source.

[jeffwidman]

Last I heard, that's no longer true now that Travis was bought by PE: https://news.ycombinator.com/item?id=25338983

Azure pipelines, Github Actions, or CircleCI are common choices. Given the simplicity of what we need, GitHub Actions is probably both simple while still powerful enough to do what we want.

Just need to wire it up.