Upgrade compatibility tests to stretch. (#7185)
Upgrade compatibility tests to stretch. (#7185)

Inspired by #7180, there's no reason for these tests to be running on old stable. This upgrades them to the latest stable version of Debian.

You can see tests passing with these changes at https://travis-ci.com/certbot/certbot/builds/116844923.
stweil/letsencrypt,letsencrypt/letsencrypt,lmcro/letsencrypt,stweil/letsencrypt,letsencrypt/letsencrypt,lmcro/letsencrypt