Upgrade protobuf to v3.6.1

Merge commit 'be9845eb2abf508c86da76321b4e2c352b45b532' into master

Change-Id: I1b3cd2e177686afe95f44abc4fb0c6f9e001e784
diff --git a/third_party/bazel/BUILD b/third_party/bazel/BUILD
index 4aecbea..ed1724a 100644
--- a/third_party/bazel/BUILD
+++ b/third_party/bazel/BUILD
@@ -18,5 +18,5 @@
         "protos/crosstool_config_pb2.py",
     ],
     visibility = ["//visibility:public"],
-    deps = ["//third_party/protobuf:protobuf_python"],
+    deps = ["@com_google_protobuf//:protobuf_python"],
 )