Use module_load_include instead of require_once

We should probably use module_load_include instead of require_once as indicated here:

http://mediatribe.net/en/node/78
http://buildamodule.com/blog/the-correct-way-to-include-files-in-your-dr...
https://api.drupal.org/api/drupal/includes%21module.inc/function/module_...

Hope that helps

Note: this is for the 7.x-1.x-caching branch