Comment 18 for bug 1278863

For those who don't have aptitude or ppa-purge, the problem can be solved (in a dirty way) by downloading the .deb package and installing it through the command: dpkg --force-overwrite -i <package_name>.
Removing gnome PPA had no effects for me, because they were already automatically disabled through the do-release-upgrade command.
The suggested command will enable you to use apt-get again, but can be harmful to your system. So use it at your own risk.