Update Vagrantfile default box URL

Solves:

    default: Downloading: https://opscode-vm-bento.s3.amazonaws.com/vagrant/virtualbox/opscode_ubuntu-12.04_provisionerless.box
An error occurred while downloading the remote file. The error
message, if any, is reproduced below. Please fix this error and try
again.

The requested URL returned error: 404 Not Found
Merge pull request #45 from jpic/patch-1

Update Vagrantfile default box URL