display version by default when listing boxes

example usage:

$ vagrant box list
berendt/ubuntu-14.04-amd64 (virtualbox) [0.1.3]
berendt/ubuntu-14.04-amd64 (virtualbox) [0.1.4]
berendt/ubuntu-14.04-amd64 (virtualbox) [0.1.5]

related to issue #3313