Bump tensorflow-gpu from 1.15.2 to 1.15.4 in /tf (#203)

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] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

tf/requirements-gpu.txt

@@ -4,5 +4,5 @@ numpy==1.16.4
 pandas==0.23.4
 scikit-learn==0.19.2
 scipy==1.1.0
-tensorflow-gpu==1.15.2
+tensorflow-gpu==1.15.4
 requests