Update box_url pattern to use newer Opscode Bento S3 bucket

With commit https://github.com/opscode/bento/commit/3adf574c33d90cec1dcc2edad41354ebdb6b24d4
the documentation for Opscode's Bento project switched from s3://opscode-vm
to s3://opscode-vm-bento for base box images, citing a change in their AWS
infrastructure management strategy as reason for the change.