Fixes Vagrant image URL to download.

URL for current builds of Ubuntu Trusty server image http://cloud-images.ubuntu.com/vagrant/trusty/current/ only contains a file for 32 bits platform. I have changed the server image URL to http://cloud-images.ubuntu.com/vagrant/trusty/trusty-server-cloudimg-amd64-juju-vagrant-disk1.box since it is very easy to find (no other judgement was applied to select this box).