fix demo server libc6 version

it has a newer version than is in wheezy, this is not good because it causes libssl-dev to fail to install due to dependencies on wheezy libc6-dev (and probably other problems). Need to revert that back to the wheezy version and then resnapshot the images

(from redmine: created on 2013-09-05, closed on 2013-09-05)