Bump tensorflow from 1.15.0 to 1.15.2

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.0 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@v1.15.0...v1.15.2)

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

requirements.txt

@@ -1,4 +1,4 @@
-tensorflow==1.15.0
+tensorflow==1.15.2
 dynet==2.1
 torch==1.3.1
 transformers==2.3.0