generalise imports in read_activations

keras_visualize_activations/read_activations.py

 import keras.backend as K
 
-from keras.engine.input_layer import InputLayer
-from keras.layers.core import Masking
+from keras.layers import Masking, InputLayer
 
 def get_activations(model, model_inputs, print_shape_only=False, layer_name=None):
     print('----- activations -----')