Bump dockerflow from 2019.5.0 to 2019.6.0
Bump dockerflow from 2019.5.0 to 2019.6.0

Bumps [dockerflow](https://github.com/mozilla-services/python-dockerflow) from 2019.5.0 to 2019.6.0.
- [Release notes](https://github.com/mozilla-services/python-dockerflow/releases)
- [Changelog](https://github.com/mozilla-services/python-dockerflow/blob/master/docs/changelog.rst)
- [Commits](https://github.com/mozilla-services/python-dockerflow/compare/2019.5.0...2019.6.0)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
Kinto/kinto-attachment