Bump tensorflow from 1.7.1 to 1.15.4

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.7.1 to 1.15.4.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.7.1...v1.15.4)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -77,7 +77,7 @@ seaborn==0.9.0
 Send2Trash==1.5.0
 six==1.12.0
 tensorboard==1.7.0
-tensorflow==1.7.1
+tensorflow==1.15.4
 termcolor==1.1.0
 terminado==0.8.1
 testpath==0.4.2