run-unit-test-docker.sh: Update to ubuntu:artful

Move with the times and also avoid C++ standard library bugs that caused
patches to phosphor-mboxd to fail CI[0].

[0] https://github.com/openbmc/openbmc/issues/2986

Closes openbmc/openbmc#2986

Tested: Ran `DISTRO=ubuntu:artful ./test-distro.sh` with a set of
patches against repositories that were failing tests. The patches are
now in Gerrit. All tests passed.

Change-Id: I181d5f5e0fec7c070cd06cc60a54134e2ed1c877
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
1 file changed