Comment 2 for bug 1310816

Your /etc/apt/sources.list file only has entries for the main component of the archive with http://mirrors.digitalocean.com/ubuntu/ which is not an official mirror of Ubuntu. Subsequently, the upgrade was unable to find the essential package ubuntu-minimal. You'll need to either choose a different mirror or only use mirrors.digitalocean.com entries in your sources.list file and then respond Yes to the question about using an unofficial mirror and rewriting sources.list.