Bump tensorflow from 1.15.0 to 1.15.4

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.0 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@v1.15.0...v1.15.4)

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

Author: dependabot[bot]

requirements.txt

@@ -52,7 +52,7 @@ scipy==1.3.3
 six==1.13.0
 tb-nightly==1.15.0a20190806
 tensorboard==1.15.0
-tensorflow==1.15.0
+tensorflow==1.15.4
 tensorflow-estimator==1.15.1
 tensornets==0.4.6
 termcolor==1.1.0