Ensure `total_premiums` always returns a `Money`

A bug occurred in certain scenarios where `premiums` would return
an empty array, so `sum`ming it would return a `Fixnum` instead of
a `Money` object. The `state_aid` calculation always expects a `Money`
object, so it blew up when it didn't have one for `total_premiums`.
Merge pull request #181 from alphagov/fix-premium-schedule-bug

Ensure total_premiums always returns a Money