Bump apache-beam[gcp] from 2.15.0 to 2.18.0 (#199)

Bumps [apache-beam[gcp]](https://github.com/apache/beam) from 2.15.0 to 2.18.0.
- [Release notes](https://github.com/apache/beam/releases)
- [Changelog](https://github.com/apache/beam/blob/master/CHANGES.md)
- [Commits](https://github.com/apache/beam/compare/v2.15.0...v2.18.0

)

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

requirements.prereq.txt

-apache_beam[gcp]==2.15.0
+apache_beam[gcp]==2.18.0
 tensorflow==1.15.2