message: Update install_pip_packages.sh to protobuf 3.6.1
diff: @@ -60,8 +60,8 @@ pip2 install --upgrade markdown==2.6.8
  pip3 install --upgrade markdown==2.6.8
  # Install protobuf.
  -pip2 install --upgrade protobuf==3.6.0
  -pip3 install --upgrade protobuf==3.6.0
  +pip2 install --upgrade protobuf==3.6.1
  +pip3 install --upgrade protobuf==3.6.1
  # Remove obsolete version of six, which can sometimes confuse virtualenv.
  rm -rf /usr/lib/python3/dist-packages/six*
  