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