You can have the concrete company put warm water in the truck, and then rent some concrete blankets to cover it after you pour, or like you say, you could try wrapping the building in plastic and putting one of those propane-powered heaters inside.

Honestly, if it were me, I would just lay down the gravel now and wait a couple of weeks until it warms up to pour the concrete. WAY easier than pouring in the freezing cold and trying to keep it warm.