Update third_party/protobuf to protobuf-v3.0.0-beta-3

third_party/protobuf/python/README.md

Index: third_party/protobuf/python/README.md
diff --git a/third_party/protobuf/python/README.md b/third_party/protobuf/python/README.md
index 1b5b9dffd9a1ad209293b428197cbfad31269597..57acfd94d688b6edba4eab9ab0e1f65b5548476c 100644
--- a/third_party/protobuf/python/README.md
+++ b/third_party/protobuf/python/README.md
@@ -123,13 +123,5 @@ C++ Implementation
 The C++ implementation for Python messages is built as a Python extension to
 improve the overall protobuf Python performance.
 
-To use the C++ implementation, you need to:
-1) Install the C++ protobuf runtime library, please see instructions in the
-   parent directory.
-2) Export an environment variable:
-
-    $ export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp
-
-You must set this variable at runtime, before running your program, otherwise
-the pure-Python implementation will be used. In a future release, we will
-change the default so that C++ implementation is used whenever it is available.
+To use the C++ implementation, you need to install the C++ protobuf runtime
+library, please see instructions in the parent directory.