Pin mujoco-py to <1.0.0

We'll upgrade Gym to use it at a later date, however that will likely involve bumping all the envs, which we're trying to avoid for now.

setup.py

@@ -11,7 +11,7 @@ extras = {
   'board_game' : ['pachi-py>=0.0.19'],
   'box2d': ['Box2D-kengz'],
   'classic_control': ['PyOpenGL'],
-  'mujoco': ['mujoco_py>=0.4.3', 'imageio'],
+  'mujoco': ['mujoco_py<1.0.0,>=0.4.3', 'imageio'],
   'parameter_tuning': ['keras', 'theano'],
 }