Use go modules, upgrade dependencies, add skaffold.dev

[jorik]

Hey @chuckha & @timothysc,

This pull requests is based on the work as described in this comment: https://github.com/heptiolabs/eventrouter/issues/77#issuecomment-522000759 What's included, in short:

• Use go modules instead of dep
• Use a multistage Dockerfile to build/test the project
• Updated the versions of:
  • Go: v1.9 -> v1.12.x
  • Alpine Linux: v3.6 -> v3.9.
  • Go dependencies, for example: k8s.io/client_go v5.0.0 -> v12.0.0

I tried to find an Alpine Linux 3.9 & Go 1.12.x on gcr.io/heptio-images/...., but couldn't find any. So I'm using the publicly available ones now.

I've also added a skaffold.yaml (using skaffold) to easily run the EventRouter in your local Kubernetes cluster using the already provided eventrouter.yaml. I hope this is ok.

I understand that the size of this PR can be quite concerning, but it actually deletes a lot more than it adds. I hope we can find a way to merge these changes into the upstream branch, as we would like to contribute to your repository!

If these changes will be accepted, I will create a new PR soon that actually contributes the new sink.

Thanks for the cool open source project!

Jorik

[vigith]

go 1.13, we have module mirror, may be we should ditch vendoring all together?