[efficientnet/model.py] Fix TensorFlow is_keras_tensor AttributeError

[SamuelMarks]

Running TensorFlow 1.14.0 with your model, and got this error:

   File "/opt/venvs/tflow3.6.8/lib/python3.6/site-packages/ml_glaucoma/models/efficientnet.py", line 45, in efficient_net
     input_tensor=inputs, **kwargs).outputs
   File "/opt/venvs/tflow3.6.8/lib/python3.6/site-packages/efficientnet/__init__.py", line 57, in wrapper
     return func(*args, **kwargs)
   File "/opt/venvs/tflow3.6.8/lib/python3.6/site-packages/efficientnet/model.py", line 548, in EfficientNetB4
     **kwargs)
   File "/opt/venvs/tflow3.6.8/lib/python3.6/site-packages/efficientnet/model.py", line 369, in EfficientNet
     if not backend.is_keras_tensor(input_tensor):
   File "/opt/venvs/tflow3.6.8/lib/python3.6/site-packages/tensorflow/python/util/deprecation_wrapper.py", line 106, in __getattr__
     attr = getattr(self._dw_wrapped_module, name)
 AttributeError: module 'tensorflow.python.keras.api._v1.keras.backend' has no attribute 'is_keras_tensor'

Solved by using is_keras_tensor from elsewhere in tensorflow module.

[qubvel]

Thanks!