Inconsistent public path in rake task and railtie

I think we should use `config.paths['public'].first` in `output` method to make it consistent with `manifest_assets_path` in
[sprockets-rails/lib/railtie.rb](https://github.com/rails/sprockets-rails/blob/master/lib/sprockets/railtie.rb#L70)
Merge pull request #161 from MQuy/patch-1

Inconsistent public path in rake task and railtie