[-] Installer : Fix require error on case sensitive file systems

On case sensitive file systems, the `require_once` call will fail because functions names are camel cased but filenames in `install/upgrade/php/` are in lower case.

This patch avoid errors like this one:
`Warning: require_once(/var/www/example.com/install/upgrade/php/setAllGroupsOnHomeCategory.php) [function.require-once]: failed to open stream: No such file or directory in /var/www/example.com/install/upgrade/upgrade.php on line 349`