build(deps): bump tensorflow from 1.15.2 to 1.15.4 in /mot

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] <support@github.com>

Author: dependabot[bot]

mot/requirements.txt

@@ -1,4 +1,4 @@
-tensorflow==1.15.2
+tensorflow==1.15.4
 opencv_python
 scikit-image
 scikit-learn