Bump tensorflow from 1.7.1 to 1.15.2

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.7.1 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@v1.7.1...v1.15.2)

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

Author: dependabot[bot]

requirements.txt

@@ -101,7 +101,7 @@ simplegeneric==0.8.1
 six==1.11.0
 sparkmagic==0.12.5
 tensorboard==1.7.0
-tensorflow==1.7.1
+tensorflow==1.15.2
 tensorflow-gpu==1.15.2
 termcolor==1.1.0
 terminado==0.8.1