Legal actions mask bug

[jhtschultz]

Found a bug in epsilon_greedy() in policy_ops.py when applying legal_actions_mask. It fails when masking the action with the highest action value.

For example:

action_values = [2.0, 1.0, 1.0]
legal_actions_mask = [0., 1., 1.]
epsilon = 0.1
result = policy_ops.epsilon_greedy(action_values, epsilon, legal_actions_mask).probs

Outputs:
[0.9 0.05 0.05]