tested and broken due to CentOS 6.x virtualenv path not being found
Error due to path to virtualenv not found by letsencrypt-auto. Issue submitted at https://github.com/letsencrypt/letsencrypt/issues/2300 and outlined at Client Version 0.3.0
rpm -ql python27-virtualenv | grep bin
/usr/bin/virtualenv-2.7
full error output at https://gist.github.com/centminmod/6ce4253f3075c25092a8