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.

Example from before the change:
![screenshot from 2014-04-30 16 22 55](https://cloud.githubusercontent.com/assets/764390/2853969/fa1faf58-d14a-11e3-8f6c-545593ad5f00.png)

Calling order.update_totals ensures that the total is correct.