Bump tensorflow from 1.13.1 to 1.15.4 in /docker

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.13.1 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@v1.13.1...v1.15.4)

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Author: dependabot[bot]

docker/requirements.txt

@@ -13,7 +13,7 @@ requests
 scipy
 sklearn
 socketIO-client-nexus
-tensorflow==1.13.1
+tensorflow==1.15.4
 uvicorn
 jsonschema