Update Debian CI build to use Stretch
The Debian CI build should be updated to run under Stretch
There will be maintenance performed on git.ligo.org, chat.ligo.org, containers.lig.org, and docs.ligo.org starting at 9am PDT on Tuesday 18th August 2020. There will be an extremely small period of downtime at the start of the maintenance window as various services are restarted. Please address any comments, questions, or concerns to firstname.lastname@example.org.