Bump tensorflow from 1.15.0 to 1.15.2 in /tf (#152)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.0 to 1.15.2.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](https://github.com/tensorflow/tensorflow/compare/v1.15.0...v1.15.2)

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tf/requirements-cpu.txt

@@ -4,5 +4,5 @@ numpy==1.16.4
 pandas==0.23.4
 scikit-learn==0.21.2
 scipy==1.3.0
-tensorflow==1.15.0
+tensorflow==1.15.2
 requests