same question:
http://askubuntu.com/questions/802154/software-updater-does-not-inform-me-of-ubuntu-16-04
http://askubuntu.com/questions/801980/how-do-i-upgrade-from-14-04-to-16-04-1
http://askubuntu.com/questions/801757/waiting-to-upgrade-14-04-4-lts-to-16-04-1-lts