Bump tensorflow from 1.15.2 to 1.15.4

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.2 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@v1.15.2...v1.15.4)

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

Author: dependabot[bot]

requirements.txt

@@ -29,7 +29,7 @@ scipy==1.2.1
 six==1.12.0
 sklearn==0.0
 tensorboard==1.13.1
-tensorflow==1.15.2
+tensorflow==1.15.4
 tensorflow-estimator==1.13.0
 termcolor==1.1.0
 werkzeug==0.15.3