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/scripts/format-code.sh b/scripts/format-code.sh
index 42be8ec..b666ebe 100755
--- a/scripts/format-code.sh
+++ b/scripts/format-code.sh
@@ -26,7 +26,7 @@
 
 if [ -f ".clang-format" ]; then
   find . -regextype sed -regex ".*\.[hc]\(pp\)\?" -not -name "*mako*" -print0 |\
-     xargs -0 "clang-format-5.0" -i
+     xargs -0 "clang-format-6.0" -i
   git --no-pager diff --exit-code
 fi