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Debian stretch was just released, so `debian:testing` and
`debian:stetch` are starting to diverge; this commit keeps the travis-ci
docker image on stretch for gcc6 and clang3.9.

Debian has also moved gcc 7 from experimental to unstable, so this
switches the gcc7 build to `sid`.  Once it migrates to `testing` I'll
switch the gcc 7 build docker image to `testing` and take it out of
failure-allowed.