Bump tensorflow from 1.13.1 to 1.15.2 in /src

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.13.1 to 1.15.2.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.13.1...v1.15.2)

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

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==1.13.1
+tensorflow==1.15.2
 flask_cors==3.0.8
 simplejson==3.16.0