Stop the "pop" between audio tracks

As noted in https://github.com/RuggedCircuits/Rugged-Audio-Shield-Library/issues/1, I was getting an audible "pop" in the speakers between wav tracks. I noticed that I didn't get the "pop" if I manually stopped the track using RAS.Stop(); before the track finished. This patch manually calls the play_stop() function and fixes the "pop" for me.