DRY up Board#robbed_hex logic

No need to store it as an instance variable; we can infer it by examining our hexes.
remove unused @longest_road_player setting

another former instance variable that wasn't necessary
remove Board.new; just define #initialize better. Allow configuration of side length and # of players.
add comment to unclear longest road logic
move test/run.rb to just test.rb

Makes a little more sense...