The box 'chef/debian-7.6' could not be found.

```
Bringing machine 'default' up with 'virtualbox' provider...
There are errors in the configuration of this machine. Please fix
the following errors and try again:

vm:
* The box 'chef/debian-7.6' could not be found.
```

You should add in the Vagrantfile:

``` ruby
config.vm.box_url = 'https://atlas.hashicorp.com/chef/boxes/debian-7.6/versions/1.0.0/providers/virtualbox.box'
```
