added a parent method to contain the returning value of the minimax recursive formula

rewrote recursive formula to be a child of another method and renamed it #determine_value
#eval_options is now the parent method that will determine the best placement from the returning values of #determine_value
wrote #gameover? method to return a boolean, checks both players for a win, or tie
wrote #value to return a score of a winning party
wrote rspec tests to check said methods