Util#find_player_classes is order dependent

`(methods & PLAYER_METHODS) == PLAYER_METHODS` at [util.rb:13]
depends on `methods` containing `PLAYER_METHODS` in the exact order defined on [util.rb:5](https://github.com/threedaymonk/battleship/blob/master/lib/battleship/util.rb#L5):
`[:name, :new_game, :take_turn]`

Instead `(PLAYER_METHODS-methods).empty?` should be used as it's not order dependent.

Semi-related: I have failing tests, even on an un-modified master branch: [gist](https://gist.github.com/BM5k/93cf8b29d5089b495da9).