Correct private company corner case

There was a bug I discovered in 1846's Mail Contract which could result in a route being selected worth less than the best route. The situation is very rare in actual game play, but not impossible. And as I was able to adjust the algorithm without greatly increasing the runtime, it was worth fixing. See #32 for more details.

The change which necessitated a minor version increment was creating and returning RouteSet objects. They act like a list (the previous return type) in many ways, but not all.