Bump buildifier version to 0.28.0

Change-Id: I8f31be0f523a3972241744a017e45f8687486291
diff --git a/jenkins-docker/slave-bazel-nocache/Makefile b/jenkins-docker/slave-bazel-nocache/Makefile
index 83111ee..c10970c 100644
--- a/jenkins-docker/slave-bazel-nocache/Makefile
+++ b/jenkins-docker/slave-bazel-nocache/Makefile
@@ -1,7 +1,7 @@
 NO_CACHE=false
 IMAGE=gerritforge/gerrit-ci-slave-bazel:debian-stretch-nocache
 BAZEL_VER=0.29.0
-BUILDIFIER_VER=0.26.0
+BUILDIFIER_VER=0.28.0
 BAZELISK_VER=0.0.7
 NODE_VER=10
 WCT_VER=6.8.0