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/grpc-ecosystem/grpc-gateway/utilities/doc.go

@@ -0,0 +1,2 @@
+// Package utilities provides members for internal use in grpc-gateway.
+package utilities