lib/game_of_life/board.rb
def neighbors_of_cell_at(x, y)
      neighbors = coords_of_neighbors(x, y).map { |x, y| self.cell_at(x, y) }
      neighbors.reject {|n| n.nil?}
    end
def neighbors_of_cell_at(x, y)
      neighbors = coords_of_neighbors(x, y).map { |x, y| self.cell_at(x, y) }
      neighbors.reject {|n| n.nil?}
    end
Finds the neighbors of a given {Cell}'s co-ordinates. The neighbors are the eight cells
 that surround the given one.
 @param [Integer] x the x-coordinate of the cell you find the neighbors of
 @param [Integer] y the y-coordinate of the cell you find the neighbors of
 @return [Array<Cell>]