For Trusty (14.04)

$ apt-get install apt-transport-https
$ echo 'deb https://rex.linux-files.org/ubuntu/ trusty rex' >> /etc/apt/sources.list
$ wget -O - https://rex.linux-files.org/DPKG-GPG-KEY-REXIFY-REPO | apt-key add -
$ apt-get update
$ apt-get install rex


install REX :
http://rexify.org/get.html

documentation REX :

http://rexify.org/docs/guides/start_using__r__ex.html