Aruba.it has a strict write permission on his web hosting servers.
Just adding the following two simple lines at the top of the wordpress configuration file wp-config.php and your website will be able to run all the updates without returning any error.
<?php
define('FS_CHMOD_FILE',0755);
define('FS_CHMOD_DIR',0755);
...
?>Enjoy your automatic wordpress upgrades and plugin updates.