removed `frontend` call that was causing NoMethodError

During the checkout flow, after putting in address information (i.e. upon hitting the "delivery" step), our Spree + spree_bootstrap_frontend app craps out saying "NoMethodError in Spree::Checkout#edit" and "undefined method `frontend' for #<ActiveRecord::Associations::CollectionProxy::ActiveRecord_Associations_CollectionProxy_Spree_ShippingRate:0x007ff24e536c98>"

After removing the call to `frontend` at line 44 of the _deliver.html.erb partial, the checkout flow works fine. I'm not sure what the frontend call is used for, maybe I'm missing something? Please let me know; I'm new to Spree!

Thanks for putting Bootstrap on Spree. It makes our app waaaay easier to customize.