Guzzle 6 autoload functions.php

I'm trying to update to Guzzle 6 and in the changelog you have this:

Static functions in GuzzleHttp\Utils have been moved to namespaced functions under the GuzzleHttp namespace. This requires either a Composer based autoloader or you to include functions.php.

Now I have tried different ways of autoloading the functions but I'm getting different errors. What is the proper way of autoloading the functions on composer.json

via Chebli Mohamed