Bump tensorflow in /examples/trials/network_morphism (#2926)

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](https://github.com/tensorflow/tensorflow/compare/v1.15.2...v1.15.4

)
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examples/trials/network_morphism/requirements.txt

 numpy==1.14.2
-tensorflow==1.15.2
+tensorflow==1.15.4
 torchvision==0.2.1
 Keras==2.3.1
 torch==0.4.1