Fix exception for undefined store_url method in empty cart redirect

The [app’s routes][ar] are configured to route the root of the application to `StoreController`, but this route wasn’t given a proper name of `store` to match the `store_url` helper used elsewhere (like in `CartsController` and `LineItemsController`).

To fix this problem, simply add the `:as` option to the `root` route with the `store` value. See the [*Rails Routing from the Outside In* guide][rg] for more information.

[ar]: https://github.com/bcblackmer/Practice/blob/4fa03fefd443462d2e9ac4b94e10f36cb87802bf/depot/config/routes.rb#L13
[rg]: http://guides.rubyonrails.org/routing.html#naming-routes