Update protobuf submodule to v3.0.0-alpha-4.1
diff --git a/.gitmodules b/.gitmodules
index 2d0c009..4d35fde 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -8,7 +8,7 @@
 [submodule "third_party/protobuf"]
 	path = third_party/protobuf
 	url = https://github.com/google/protobuf.git
-	branch = v3.0.0-beta-1
+	branch = v3.0.0-alpha-4.1
 [submodule "third_party/gflags"]
 	path = third_party/gflags
 	url = https://github.com/gflags/gflags.git