Merge pull request #6 from suetfei/protobuf_update

Update protobuf to a newer version.
diff --git a/third_party/protobuf b/third_party/protobuf
index d0bfd52..5500c72 160000
--- a/third_party/protobuf
+++ b/third_party/protobuf
@@ -1 +1 @@
-Subproject commit d0bfd5221182da1a7cc280f3337b5e41a89539cf
+Subproject commit 5500c72c5b616da9f0125bcfab513987a1226e2b