🔥 Support new prerelease scheme

[carolynvs]

Prereleases are no longer consistently listed in the github tags/releases. So attempts to install 17.07.0-ce-rc1 fails because it's not "official" but the binaries do exist under https://download.docker.com/mac/static/test/x86_64/.

• Since they are cutting releases without tagging, we need to stop validating a version by looking it up on github. The true test of if a version exists is if the binary is present. Never check a version before installing #181
• We need to fix dvm list-remote --pre to list what's in the test releases directory. I think it's safe at this point to not list older prereleases from the original download location. Fix ls-remote #182
• Check for versions first in stable when installing, and fall back to test (see comment below).

[carolynvs]

Prereleases no longer always have the -rc suffix anymore. So we do see a version like 17.10.0-ce in the prerelease location, and eventually when it is "finalized", it is published to the stable location (the old test release is not removed, but should no longer be used once a stable release is available). 🤦‍♀️

[carolynvs]

Fixed by #182, #183, and #181.