Fixed issue where files method on CookbookObject is occasionally overwritten.

When running a `berks upload` I was getting the following error:

```
/home/jordan/.rvm/gems/ruby-1.9.3-p392/gems/ridley-0.11.1/lib/ridley/chef_objects/cookbook_object.rb:135:in `download_file': undefined method `find' for nil:NilClass (NoMethodError)
```

Tracked it down to this line in the code, seems that the 'files' method is occasionally getting replaced with nil. Changing the name of the 'files' variable seems to resolve the issue, so that's what I've done.