Update variants checkbox code on product page for Spree::OrdersController#populate API change

Issue #102 covered updating the slider to the new Spree::OrdersController#populate API, but the product page wasn't updated along with it. The radio buttons need to have the name "variant_id" for the submission to go through successfully. This change makes that happen.

Tested with up to date 2-2-stable of spree and spree_fancy.