Fix template for Packer 0.5.0 and Ubuntu 12.04.4
Fix template for Packer 0.5.0 and Ubuntu 12.04.4

Packer 0.5.0 renamed the virtualbox builder
https://github.com/mitchellh/packer/blob/master/CHANGELOG.md#050-12302013

Ubuntu image link to 12.04.3 was broken, updated to 12.04.4

njg/packer-vagrant-linux