Increase max_steps in DoorKey to make it learnable (#15)

gym_minigrid/envs/doorkey.py

@@ -9,7 +9,7 @@ class DoorKeyEnv(MiniGridEnv):
     def __init__(self, size=8):
         super().__init__(
             grid_size=size,
-            max_steps=4*size*size
+            max_steps=10*size*size
         )
 
     def _gen_grid(self, width, height):