Update model version number for TensorFlow + Keras training

Training Fashion-MNIST model using TensorFlow and Keras as explained at

Running the training gives:

```
Test accuracy: 0.885599970818
Traceback (most recent call last):
File "mnist.py", line 89, in
main()
File "mnist.py", line 86, in main
export_model(model, args.model_export_path)
File "mnist.py", line 69, in export_model
outputs={t.name:t for t in model.outputs})
File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/util/deprecation.py", line 324, in new_func
return func(*args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/saved_model/simple_save.py", line 83, in simple_save
b = builder.SavedModelBuilder(export_dir)
File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/saved_model/builder_impl.py", line 425, in __init__
super(SavedModelBuilder, self).__init__(export_dir=export_dir)
File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/saved_model/builder_impl.py", line 100, in __init__
"directory: %s" % export_dir)
AssertionError: Export directory already exists. Please specify a different export directory: s3://eks-ml-example/mnist/export/1
```

This error message should not come. Instead the model should be persisted with a new version number.

@Jeffwan will look into this