G'day Joey.

In [Docker.hs, line 73](https://git.joeyh.name/index.cgi/propellor.git/tree/src/Propellor/Property/Docker.hs?h=5.4.0#n73), docker.io is listed as the package to be installed.

Docker.installed currently fails for me on Stretch with:

    E: Package 'docker.io' has no installation candidate

Unless I'm mistaken, from Stretch this is now replaced by "docker-engine".
