Update keras code

[pyaf]

Hi,
I'm going through your code, I see some code which has been deprecated in due course of time.
like here
visualize_util has been changed to vis_utils and plot to plot_model.

I would like to fix such bugs by raising a PR, let me know your consent.

[dcorti]

Hi
Yes, keras keeps updating and therefore many parts of the become obsolete.
Please go ahead with the PR, thank you

[ZexuanTHU]

@pyaf I clone this repo and try to work through the training process, there do have several warnings and even rejects under the latest Keras environment. About the plot_model issue, following is a solution. I think just simply remove the import and references of plot_model from /src/retinaNN_training.py will work.