add env.close method (#100)

as mentioned here: https://github.com/openai/gym/issues/1056#issuecomment-471136134 

close=True was removed a while ago, so this change just adds an 'env.close' method exactly the same as the way you were doing it, without removing the old way, so it's hopefully backwards compatible (let's test it!)

gym_minigrid/minigrid.py

@@ -1293,3 +1293,8 @@ class MiniGridEnv(gym.Env):
             self.window.set_caption(self.mission)
 
         return img
+
+    def close(self):
+        if self.window:
+            self.window.close()
+        return