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/any.go

@@ -312,6 +312,10 @@ func (codec *directAnyCodec) Decode(ptr unsafe.Pointer, iter *Iterator) {
 
 func (codec *directAnyCodec) Encode(ptr unsafe.Pointer, stream *Stream) {
 	any := *(*Any)(ptr)
+	if any == nil {
+		stream.WriteNil()
+		return
+	}
 	any.WriteTo(stream)
 }