Bump tensorflow-gpu from 1.15.2 to 1.15.4

Bumps [tensorflow-gpu](https://github.com/tensorflow/tensorflow) from 1.15.2 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@v1.15.2...v1.15.4)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -66,7 +66,7 @@ six==1.12.0
 sklearn==0.0
 sympy==1.4
 tensorboard==1.14.0
-tensorflow-gpu==1.15.2
+tensorflow-gpu==1.15.4
 tensorflow-estimator==1.14.0
 termcolor==1.1.0
 terminado==0.8.2