add a guard to the build agent service start

- [ ] @sneal 
We have newer Ubuntu base boxes we've built with Packer
`http://vagrantboxes.hq.daptiv.com/vagrant/boxes/virtualbox_ubuntu-12.04_chef-11.12.4_1.0.5.box`

These boxes (we have both virtualbox and vmware) have all updates applied and apt-get update.