fixes add-fields to allow select (vs. just input)

I'm not sure if this belongs in spree or just this extension, but the add-fields javascript https://github.com/spree/spree/blob/1-3-stable/core/app/assets/javascripts/admin/admin.js.erb#L160 only handles 'input', which means the pricing select box doesn't get the right id, which causes the 'add' to fail.