Support non-string cookbook_paths

Spork blows up if the first cookbook_path in knife.rb isn't a string:

    runner.rb:17:in `spork_config': undefined method `gsub' for #<Pathname:0x00000002f90e28> (NoMethodError)

In my case the first cookbook_path is `Librarian::Chef.install_path()`.

This PR makes sure that the cookbook_path in spork is always a string.