Using dockerd as docker daemon has been deprecated

[ageekymonk]

Fix for #22

[mongrelion]

@marcusianlevine before you merge this could you please make sure that it doesn't break any previous supported version that we offer?
I know this sucks to have to manually test this. We really need those integration tests ;_;

[marcusianlevine]

I ran the build locally and everything worked fine, but as-is this change will break backwards compatibility with versions of Docker <1.13

Do we intend to support 1.12 and below? For reference, Rancher's setup scripts support down to 1.10

If so, we need to add a conditional that checks the Docker version and decides which daemon command to use.