Update metadata.rb

Following command occurs an error.
```sh
% bundle exec berks vendor cookbooks
Resolving cookbook dependencies...
Fetching 'chef-solr' from git://github.com/theozaurus/chef-solr.git (at master)
Ridley::Errors::MissingNameAttribute The metadata at '/var/folders/6m/4rjdzzhj15dd_tgwdd70szc00000gn/T/d20140526-46134-1p2dzww' does not contain a 'name' attribute. While Chef does not strictly enforce this requirement, Ridley cannot continue without a valid metadata 'name' entry.
```