I had problems too on updating 7.14 to 7.15 with apache and php-fpm, the 7.15 update ran into an timout because it took a little bit too long. You may have to wait more than 5 minutes for the update to complete.
Had to increase the timout in /etc/apache2/apache2.conf
#
# Timeout: The number of seconds before receives and sends time out.
#
Timeout 900