Fix mobilenet error

src/nets/mobilenet.py

@@ -1,7 +1,7 @@
 from keras import backend as K
 from keras.layers import (Activation, BatchNormalization, Conv2D,
                           DepthwiseConv2D, Dropout, GlobalAveragePooling2D,
-                          Input, Reshape)
+                          Input, Reshape, ReLU)
 from keras.models import Model
 
 
@@ -98,7 +98,7 @@ def MobileNet(input_shape=None,
     return model
 
 def relu6(x):
-    return K.relu(x, max_value=6)
+    return ReLU(x, max_value=6)
 
 if __name__ == '__main__':
     model = MobileNet(input_shape=(224, 224, 3))