Minor bug for frame_skip at AtariPreprocessing

[alirahkay]

When setting frame_skip=1 in the AtariPreprocessing, we will always have t=1 in the step function (line 475 of atari_lib.py). And so, self.screen_buffer[0] never gets updated.
Now considering the _pool_and_resize when frame_skip is one, transformed_image is exactly the resized self.screen_buffer[0], and so, it will never change in the entire episode.