Bump apache-beam from 2.14.0 to 2.15.0 (#138)

Bumps [apache-beam](https://github.com/apache/beam) from 2.14.0 to 2.15.0.
- [Release notes](https://github.com/apache/beam/releases)
- [Commits](https://github.com/apache/beam/compare/v2.14.0...v2.15.0

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.prereq.txt

-apache_beam[gcp]==2.14.0
+apache_beam[gcp]==2.15.0
 tensorflow==1.14.0