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/runtime/doc.go

@@ -0,0 +1,5 @@
+/*
+Package runtime contains runtime helper functions used by
+servers which protoc-gen-grpc-gateway generates.
+*/
+package runtime