Bump tensorflow from 1.15.3 to 1.15.4 (#255)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.3 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/v1.15.3...v1.15.4

)

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requirements.prereq.txt

 apache_beam[gcp]==2.24.0
-tensorflow==1.15.3
+tensorflow==1.15.4