Bazel release script: run `gsutil cp` in parallel

Run the command with the `-m` option so gsutil
will copy files in parallel.

Suggested by gsutil itself
http://ci.bazel.io/view/Bazel%20bootstrap%20and%20maintenance/job/Bazel-Release/83/console
(look for "NOTE: You are performing a sequence of
gsutil operations")

--
MOS_MIGRATED_REVID=140119304
diff --git a/scripts/ci/build.sh b/scripts/ci/build.sh
index 0c4df206a..3a5f01a 100755
--- a/scripts/ci/build.sh
+++ b/scripts/ci/build.sh
@@ -300,7 +300,7 @@
     create_index_html "${dir}/${release_name}/rc${rc}" \
         >"${dir}/${release_name}/rc${rc}"/index.html
     cd ${dir}
-    "${gs}" cp -a public-read -r . "gs://${GCS_BUCKET}"
+    "${gs}" cp -m -a public-read -r . "gs://${GCS_BUCKET}"
     cd "${prev_dir}"
     rm -fr "${dir}"
     trap - EXIT