unit-test CI: switch to clang-format-6.0

clang-format-5.0 doesn't support the IncludeBlocks configuration option.

Change-Id: I5668c46e502b64f3302754a29383f3137e321014
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/run-unit-test-docker.sh b/run-unit-test-docker.sh
index a67bb40..ef9b4c5 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=${DISTRO:-ubuntu:artful}
+DISTRO=${DISTRO:-ubuntu:bionic}
 WORKSPACE=${WORKSPACE:-$(mktemp -d --tmpdir unit-test.XXXXXX)}
 OBMC_BUILD_SCRIPTS="openbmc-build-scripts"
 UNIT_TEST_PY_DIR="scripts"