Update requirements to TensorFlow gpu 1.13.2

[Ryoji Kuwae Neto]

For anyone trying the presented model, be aware that you should first uninstall TensorFlow if you're using Colab.
And then install TensorFlow gpu 1.13.2 instead of tf 1.13.1

[robertanto]

Hi @Ryojikn , thank you for helping us!

Which problem have you encountered by using TensorFlow 1.13.1 (GPU)?