Update readme for setting up protobuf 2.6.0 on mac. (apache#4492)

* Update readme for setting up protobuf 2.6.0 on mac.
Update port for standalone.conf used for spin up server for testing.

* No longer require protobuf 2.6 specificly, so update the instruction to accommodate that.
Also leave tests/standalone.conf as original state.

pulsar-client-cpp/README.md

@@ -131,8 +131,7 @@ export OPENSSL_INCLUDE_DIR=/usr/local/opt/openssl/include/
 export OPENSSL_ROOT_DIR=/usr/local/opt/openssl/
 
 # For Protobuf
-brew tap homebrew/versions
-brew install protobuf260 boost boost-python log4cxx jsoncpp
+brew install protobuf boost boost-python log4cxx jsoncpp
 
 # For gtest
 cd $HOME