build(deps): Bump google-cloud-storage from 1.27.0 to 1.28.0 in /bot (#500)

Bumps [google-cloud-storage](https://github.com/googleapis/python-storage) from 1.27.0 to 1.28.0.
- [Release notes](https://github.com/googleapis/python-storage/releases)
- [Changelog](https://github.com/googleapis/python-storage/blob/master/CHANGELOG.md)
- [Commits](googleapis/python-storage@v1.27.0...v1.28.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

bot/requirements.txt

@@ -1,5 +1,5 @@
 -e ../tools #egg=code-coverage-tools
-google-cloud-storage==1.27.0
+google-cloud-storage==1.28.0
 libmozdata==0.1.68
 pytoml==0.1.21
 pytz==2019.3