We don't seem to have been consistent as to whether it was 3.6 or 3.7, but
we weren't testing it consistently anyway.

It feels too soon to go up to 3.10 unfortunately, but 3.6/3.7 are EOL and
3.8 is EOL next year, so let's do 3.9.

Signed-off-by: Sam James <sam@gentoo.org>