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/json-iterator/go/build.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+set -e
+set -x
+
+if [ ! -d /tmp/build-golang/src/github.com/json-iterator ]; then
+    mkdir -p /tmp/build-golang/src/github.com/json-iterator
+    ln -s $PWD /tmp/build-golang/src/github.com/json-iterator/go
+fi
+export GOPATH=/tmp/build-golang
+go get -u github.com/golang/dep/cmd/dep
+cd /tmp/build-golang/src/github.com/json-iterator/go
+exec $GOPATH/bin/dep ensure -update