Bump tensorflow-gpu from 1.10.1 to 1.15.0 in /tf

Bumps [tensorflow-gpu](https://github.com/tensorflow/tensorflow) from 1.10.1 to 1.15.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](https://github.com/tensorflow/tensorflow/compare/v1.10.1...v1.15.0)

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

tf/requirements-gpu.txt

@@ -4,5 +4,5 @@ numpy==1.14.5
 pandas==0.23.4
 scikit-learn==0.19.2
 scipy==1.1.0
-tensorflow-gpu==1.10.1
+tensorflow-gpu==1.15.0
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