Bump dockerflow from 2019.10.0 to 2020.6.0

Bumps [dockerflow](https://github.com/mozilla-services/python-dockerflow) from 2019.10.0 to 2020.6.0.
- [Release notes](https://github.com/mozilla-services/python-dockerflow/releases)
- [Changelog](https://github.com/mozilla-services/python-dockerflow/blob/master/docs/changelog.rst)
- [Commits](mozilla-services/python-dockerflow@2019.10.0...2020.6.0)

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

requirements.txt

@@ -6,7 +6,7 @@ colander==1.7.0
 colorama==0.4.3
 cornice==5.0.1
 cornice-swagger==1.0.0
-dockerflow==2019.10.0
+dockerflow==2020.6.0
 elasticsearch==7.7.1
 hupper==1.10.2
 idna==2.9