Make `vagrant up` work even if network is unavailable

[sephii]

At the moment running vagrant up even on a box that is already provisioned on a computer that doesn't have any network access will result in an error because Vagrant cannot check for box updates.

We could add the following configuration option to the Vagrantfile to prevent box update checking: config.vm.box_check_update = false. It would be of course better that Vagrant doesn't completely fail but just warns about not being able to check for updates, but that's apparently not possible.