build(deps): bump tensorflow-gpu from 1.13.1 to 1.15.2

Bumps [tensorflow-gpu](https://github.com/tensorflow/tensorflow) from 1.13.1 to 1.15.2.
- [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/v1.13.1...v1.15.2)

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

requirements.txt

@@ -1,2 +1,2 @@
 calamari-ocr==0.3.5
-tensorflow-gpu==1.13.1
+tensorflow-gpu==1.15.2