Add PPA key:

add-apt-repository ppa:tuxpoldo/roundcube

Update to /etc/apt/source.list

deb http://ppa.launchpad.net/tuxpoldo/roundcube/ubuntu trusty main
deb-src http://ppa.launchpad.net/tuxpoldo/roundcube/ubuntu trusty main

Then run

apt-get update
apt-get dist-upgrade