add env.close method

as mentioned here: openai/gym#1056 (comment) 

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 @@ def render(self, mode='human', close=False, highlight=True, tile_size=TILE_PIXEL
             self.window.set_caption(self.mission)
 
         return img
+
+    def close(self):
+        if self.window:
+            self.window.close()
+        return