Add dtype to Box initialization, fixes #868 (#1092)

Otherwise it generates warnings:

#868

gym/envs/mujoco/mujoco_env.py

@@ -46,7 +46,7 @@ class MujocoEnv(gym.Env):
         bounds = self.model.actuator_ctrlrange.copy()
         low = bounds[:, 0]
         high = bounds[:, 1]
-        self.action_space = spaces.Box(low=low, high=high)
+        self.action_space = spaces.Box(low=low, high=high, dtype=np.float32)
 
         high = np.inf*np.ones(self.obs_dim)
         low = -high