Bump tensorflow-gpu from 1.14.0 to 1.15.2

Bumps [tensorflow-gpu](https://github.com/tensorflow/tensorflow) from 1.14.0 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/v1.14.0...v1.15.2)

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

requirements.txt

@@ -1,5 +1,5 @@
 numpy
-tensorflow-gpu == 1.14.0
+tensorflow-gpu == 1.15.2
 calamari-ocr == 0.3.5
 setuptools >= 41.0.0  # tensorboard depends on this, but why do we get an error at runtime?
 click