Give unique name to downloaded box

As the default virtualbox from vagrant is called "precise64" and this may be likely to be already present in your environment the installation and provisioning will fail due to unresolved dependencies.
The dependencies are only met in the virtualbox file you provide in config.vm.box_url, so you have to assure that this one is downloaded and used and not the default precise64 from the vagrant maintainer.
Merge pull request #20 from ipf/patch-1

Give unique name to downloaded box