don't use php5.4 features on autoloaded functions.php file

Once you have guzzle included into your composer file you will load functions.php on every code execution before anything else happens.
Sadly functions.php is invalid PHP on php < 5.4. While this is not problem perse it disables you to tell users that they have to use php 5.4 instead.

Actual main code in classes is not affected as it will not be loaded on each request.