Bump tensorflow from 1.13.1 to 1.15.0 in /examples

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.13.1 to 1.15.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.13.1...v1.15.0)

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Author: dependabot[bot]

examples/requirements.txt

@@ -8,7 +8,7 @@ Pillow==6.0.0
 scikit-learn==0.21.2
 scipy==1.3.0
 tensorboard==1.13.1
-tensorflow==1.13.1
+tensorflow==1.15.0
 tensorflow-estimator==1.13.0
 termcolor==1.1.0
 torch==1.2.0