Upgrade Docker guide to account for native Mac/Windows public beta

[toolness]

Hooray, the native Docker for Mac and Windows beta is now open to the public! This will really shorten our Docker guide.

I'm not sure if we should necessarily remove the docker-machine specifics from our guide just yet, as CM mentioned that the Docker native beta actually runs noticeably slower on his system than the docker-machine version, and there are bound to be other bugs. But at the very least we might want to mention the availability of the native Beta and "deprecate" the docker-machine info.