The x86_64 job is the one we run inside Docker (as opposed to 'local',
which runs on the host). Let's update this to use Ubuntu Zesty, and
since it has clang-4 (unlike the 'local' one) it makes sense to add
a clang build.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>