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

third_party/protobuf/python/README.md

 Protocol Buffers - Google's data interchange format
 ===================================================
 
 [![Build Status](https://travis-ci.org/google/protobuf.svg?branch=master)](https://travis-ci.org/google/protobuf)
 
 Copyright 2008 Google Inc.
 
 This directory contains the Python Protocol Buffers runtime library.
 
 Normally, this directory comes as part of the protobuf package, available
@@ skipping 105 matching lines @@
 web at:
 
   https://developers.google.com/protocol-buffers/
 
 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.