Bump tensorflow from 1.15.2 to 1.15.4 in /src

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)

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Author: dependabot[bot]

src/requirements.txt

@@ -3,6 +3,6 @@ gunicorn==19.9.0
 pandas==0.23.4
 numpy==1.14.6
 Keras==2.1.4
-tensorflow==1.15.2
+tensorflow==1.15.4
 flask_cors==3.0.8
 simplejson==3.16.0