Refactoring

 - remove unnecessary test for 9 spaces
 - remove spec_helper.rb and move assert_place to board_spec.rb
 - rename check_valid_move to check_empty
Add minimax class/spec
Add first test for Minimax
Add second test for minimax
Refactor after second minimax test
Refactor to use enumerable module
Add check_game_over? method and first test
Add second test for check_game_over
Add check_winner method and first test
Add winning_combos array and test for winning
Add test for check_winner for 'o'
Refactor

I made the @winning_combos array available to the tests so I'll be
able to iterate through to test each one.  I also added an
assert_check_winner helper method.