Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/github.com/docker/spdystream/CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# Contributing to SpdyStream
+
+Want to hack on spdystream? Awesome! Here are instructions to get you
+started.
+
+SpdyStream is a part of the [Docker](https://docker.io) project, and follows
+the same rules and principles. If you're already familiar with the way
+Docker does things, you'll feel right at home.
+
+Otherwise, go read
+[Docker's contributions guidelines](https://github.com/dotcloud/docker/blob/master/CONTRIBUTING.md).
+
+Happy hacking!