Switch from dep to go mod

[br-lewis]

Switch from dep to go mod

This PR changes dcos-go to handle dependencies using go mod rather than dep.

Note: in order to avoid a dependency casing conflict (see here), this PR pins github.com/docker/docker and github.com/docker/distribution to their current latest commit on master, it won't track changes in those masters automatically but this may not be desirable for us.

Checklist

• ~80% unit test coverage?
• Updated README.md?
• Completed sections of this PR template?
• Edited all sections [IN BRACKETS]

Overview of Change

Go modules are the canonical method of dependency management now with Go 1.11. It also allows dcos-go to exist outside of $GOPATH

Affected Usage

This shouldn't affect usage of other libraries.

[gpaul]

Happy days!

[drewvanstone]

Couple small typos but otherwise looks good.

As an aside, I think moving away from vendor directory would be good at some point. The community seems to be converging on project Athens. It's basically an append only artifact store for go modules. It means we could have repeatable builds without vendor checked into code.