Add a callback function to support Node.js v8 or later

[lulzneko]

This is an extension pull request to support Node.js v10 or later.

The following error occurs when using ga-analytics with Node.js v10.

Processing without callback function of fs.writeFile() API used by module.js has been changed by upgrading Node.js.
-v6: OK, no problem
-v8: Executable and deprecated logs are output
-v10: An error occurs

Passing a callback function to the fs.writeFile() argument supports v10 and later.

Thank you for the wonderful ga-analytics product.