Bump tensorflow from 1.12.0 to 1.15.2

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.12.0 to 1.15.2.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.12.0...v1.15.2)

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

Author: dependabot[bot]

requirements.txt

@@ -1,6 +1,6 @@
 torch==1.0.1.post2
 torchtext==0.6.0
-tensorflow==1.12.0
+tensorflow==1.15.2
 Keras==2.0.0
 numpy==1.15.4
 pandas==1.0.3