Configure attributes for ohai to workaround empty node.platform_family

As the default behaviour doesn't include `node.platform_family` data, ChefSpec crashes when including `build-essential` recipe.

```
Cookbook Trace:
---------------
  /tmp/d20140115-14650-d4muwz/build-essential/recipes/default.rb:21:in `from_file'
  /tmp/d20140115-14650-d4muwz/dnsimple/recipes/default.rb:20:in `from_file'
Relevant File Content:
----------------------
/tmp/d20140115-14650-d4muwz/build-essential/recipes/default.rb:
 14:  # distributed under the License is distributed on an "AS IS" BASIS,
 15:  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 16:  # See the License for the specific language governing permissions and
 17:  # limitations under the License.
 18:  #
 19:  
 20:  begin
 21>>   include_recipe "build-essential::#{node['platform_family']}"
 22:  rescue Chef::Exceptions::RecipeNotFound
 23:    Chef::Log.warn "A build-essential recipe does not exist for the platform_family: #{node['platform_family']}"
 24:  end
 25:  
```

This tries to load `build-essential::#{node['platform_family']}` but fails when it's empty or nil.