Fix renamed inbound_nodes in CuDNN recurrent test. (keras-team#9127)

- forgotten in commit 958239c (Make private topological properties Python-private)
- rename inbound_nodes to _inbound_nodes
- BTW: tests from cudnn_recurrent_test.py are not ran on Travis CI (without GPU)

tests/keras/layers/cudnn_recurrent_test.py

@@ -295,7 +295,7 @@ def test_specify_initial_state_keras_tensor():
             output = layer(inputs, initial_state=initial_state[0])
         else:
             output = layer(inputs, initial_state=initial_state)
-        assert initial_state[0] in layer.inbound_nodes[0].input_tensors
+        assert initial_state[0] in layer._inbound_nodes[0].input_tensors
 
         model = keras.models.Model([inputs] + initial_state, output)
         model.compile(loss='categorical_crossentropy', optimizer='adam')