For xUbuntu 12.04 run the following:
Keep in mind that the owner of the key may distribute updates, packages and repositories that your system will trust (more information).
echo 'deb http://download.opensuse.org/repositories/network:/messaging:/zeromq:/release-stable/xUbuntu_12.04/ /' | sudo tee /etc/apt/sources.list.d/network:messaging:zeromq:release-stable.list
curl -fsSL https://download.opensuse.org/repositories/network:messaging:zeromq:release-stable/xUbuntu_12.04/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/network_messaging_zeromq_release-stable.gpg > /dev/null
sudo apt update
sudo apt install libmicrohttpd10