* onnx: pull Protobuf 3.5.x from GitHub
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: fix Travis build configuration issue
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: additional fixes for Travis
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: update documentation - build requirements for MacOS
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: update documentation - build requirements for Ubuntu
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>