Good results after shortening the period of your sine wave to 60 steps (from 180):

Screen Shot 2017-04-18 at 1.15.31 AM.png1166×896 86.8 KB

It might be that the long range dependency is too long for such a small model. It can learn to “fit the data” when the teacher is holding its hand, but is never trained on its own outputs, so that’s as far as it goes.