Incompatibility with the most recent releases of OpenAI Gym

Because of the use of the deprecated `gym.undo_logger_setup()` method (openai/gym@4c460ba), GA3C no longer works with the most recent versions. Using this method is no longer required as OpenAI Gym no longer modifies the global logging configuration.

I've submitted a pull request (#42) to remove the use of the deprecated method.