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/reflect_marshaler.go

@@ -93,8 +93,7 @@ func (encoder *marshalerEncoder) Encode(ptr unsafe.Pointer, stream *Stream) {
 		stream.WriteNil()
 		return
 	}
-	marshaler := obj.(json.Marshaler)
-	bytes, err := marshaler.MarshalJSON()
+	bytes, err := json.Marshal(obj)
 	if err != nil {
 		stream.Error = err
 	} else {