Apply order_required on view_that_asks_for_money

Hey Chris,
what do you think about this change? It fixes an issue I have when clients are randomly hitting the view_that_asks_for_money URL without having created an order before. The main drawback I see is that it introduces a hard depdency on two functions in django-shop. Would you be ok with merging it, or do you think the dependency should be avoided, or do you have other objections?