override correct nginx cookbook attribute

The `source` recipe currently tries to influence the version of nginx to be build from source by incorrectly overriding the `nginx.source` attribute. In the upstream cookbook, `nginx.source` is assumed to be a hash, and `nginx.source.version` is loaded from `nginx.version`attribute anyway.

See https://github.com/opscode-cookbooks/nginx/blob/1.7.0/attributes/source.rb#L24