Update google-cloud-logging to 1.15.0

Created by: pyup-bot

This PR updates google-cloud-logging from 1.13.0 to 1.15.0.

The bot wasn't able to find a changelog for this release. Got an idea?

Links

• PyPI: https://pypi.org/project/google-cloud-logging
• Repo: https://github.com/googleapis/python-logging