Bump tensorflow-gpu from 1.13.1 to 1.15.4

Bumps [tensorflow-gpu](https://github.com/tensorflow/tensorflow) from 1.13.1 to 1.15.4.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.13.1...v1.15.4)

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Author: dependabot[bot]

requirements.txt

@@ -4,7 +4,7 @@ scikit-learn
 cleverhans==3.0.1
 tensorboard==1.13.1
 tensorflow==1.13.1
-tensorflow-gpu==1.13.1
+tensorflow-gpu==1.15.4
 numpy==1.18.1
 scikit-image==0.15.0
 opencv-python>=4.1.0