Update orders totals on add to cart.

When a gift card was added to the cart, there was a state which could
occur in Spree 2.2 where the order total was not correct.

Calling order.update_totals ensures that the total is correct.
Add a feature spec for cart total.
Merge pull request #56 from gmacdougall/update-order-totals-on-add-to-cart

Update orders totals on add to cart.