Added a stub cookbooks directory so that the initial `vagrant up` after a clone works

Previously, a fresh clone of this project would cause `vagrant up` to fail with:

```
chef solo provisioner:
* Cookbook path doesn't exist: /path/to/chef-solo/cookbooks
```

This pull request adds a stub `cookbooks` directory so that this error no longer occurs on a fresh clone.

Resolves #3.

/cc @adron