Find a better way to test against dev Docker

[aanand]

Shoving the binary in the repo is really ugly, but curl'ing from https://master.dockerproject.com is unstable.

[thaJeztah]

😢 looks like something that should be solved somehow; Apart from Compose, various other projects (Swarm, Kitematic) will probably need a way to run tests against a pre-release Docker as well.

[dnephin]

Curling from get.docker.com has been working for us I think, right?

And #1783 will make this better. Going to close since I think this isn't an issue anymore

[aanand]

I'd love to automate the process of watching for new releases/RCs and opening PRs to test against them, but that's a pretty hairy yak.