Bump tensorflow in /training/xgboost/structured/base (#212)
Bump tensorflow in /training/xgboost/structured/base (#212)

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)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
Co-authored-by: Torry Yang <9eeae24d6606e4d93ea912a8d0c129e13bc8927d@users.noreply.github.com>
GoogleCloudPlatform/ai-platform-samples,GoogleCloudPlatform/ai-platform-samples