Add docs on what versions of docker and docker-compose elsy supports

[paulcichonski]

We should publish a support matrix for both docker and docker-compose versions.

To support this we also need to test each docker/compose version pair in the matrix, the tasks involved in enabling this are:

• [ ] Update TravisCI build to run parallel builds across the different versions
• [ ] Figure out how to tag specific features/scenarios of the blackbox-tests if they only work starting at a given docker/compose version; so a build that runs with an earlier version can skip the feature/scenario without failing the build.