Set the $default_routes with function

If the $default_routes needs to be overridden & it's done outside of an
function the uninstall of the module is corrupting the override file.

To test install than uninstall this module https://github.com/Ha99y/PrestaShop-modules-CleanURLs/

the file /override/classes/Dispatcher.php will get corrupted after uninstall.

File to look at:
https://github.com/Ha99y/PrestaShop-modules-CleanURLs/blob/master/cleanurls/override/classes/Dispatcher.php