Upgrade buildifier to 0.15.0

Change-Id: Idcc014350fe81f6ffc1be14e2ff55f799b13ec7c
diff --git a/jenkins-docker/slave-bazel/Dockerfile b/jenkins-docker/slave-bazel/Dockerfile
index 0de1e48..dbba301 100644
--- a/jenkins-docker/slave-bazel/Dockerfile
+++ b/jenkins-docker/slave-bazel/Dockerfile
@@ -7,12 +7,12 @@
     rm /tmp/bazel_0.14.0-linux-x86_64.deb && \
     bash -c '. set-java.sh 8 && bazel version'
 
-# Install buildifier utility (0.12.0)
+# Install buildifier utility (0.15.0)
 RUN cd /tmp && \
     bash -c '. /usr/bin/set-java.sh 8 && \
     git clone https://github.com/bazelbuild/buildtools.git && \
     cd buildtools && \
-    git checkout 0.12.0 && \
+    git checkout 0.15.0 && \
     bazel build --workspace_status_command=`pwd`/status.sh //buildifier && \
     cp bazel-bin/buildifier/linux_amd64_stripped/buildifier /usr/bin && \
     rm -rf /tmp/buildtools && \