Fix for a bug where updating software fails to work.
First, kill any instance of appstreamcli. Then do the following:
cd /tmp && mkdir asfix
cd asfix
wget https://launchpad.net/ubuntu/+archive/primary/+files/appstream_0.9.4-1ubuntu1_amd64.deb
wget https://launchpad.net/ubuntu/+archive/primary/+files/libappstream3_0.9.4-1ubuntu1_amd64.deb
sudo dpkg -i *.deb
See https://bugs.launchpad.net/ubuntu/+source/appstream/+bug/1579712 for more details.