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>
diff --git a/run-unit-test-docker.sh b/run-unit-test-docker.sh
index 659c4ab..e85661e 100755
--- a/run-unit-test-docker.sh
+++ b/run-unit-test-docker.sh
@@ -11,7 +11,7 @@
 
 # Default variables
 DOCKER_IMG_NAME="openbmc/ubuntu-unit-test"
-DISTRO=${DISTRO:-ubuntu:latest}
+DISTRO=${DISTRO:-ubuntu:artful}
 WORKSPACE=${WORKSPACE:-$(mktemp -d --tmpdir unit-test.XXXXXX)}
 OBMC_BUILD_SCRIPTS="openbmc-build-scripts"
 UNIT_TEST_PY_DIR="scripts"