Testing Ubuntu 14.04

Unclear how Mesos works on 14.04. mesos-968 seems to say it’s not great
+ comments seem to say its python 2.x related, but this just a hack to
see how things go, see if it’s worth dropping it in as a replacement to
at least see what the issues are, when running on ubuntu 14.04, we
override download_url to
pull the latest RC