Switch to go modules.

[aveshagarwal]

We need to look into if we could start using go modules for dependency management.

[damemi]

I had this as a sub-task under the last k8s bump (#187). I've been looking into it and trying to get this started but I run into issues when it depends on k8s.io/kubernetes. I ended up opening #200 to investigate if we can break some of these dependencies

[aveshagarwal]

#200 (comment)

[damemi]

@aveshagarwal we should also tag a latest release, or start following upstream's release cycle, since the latest semver tag we have is https://github.com/kubernetes-sigs/descheduler/tree/v0.8.0 (last commit 2018)

[seanmalloy]

/kind cleanup