Required version of ProtocolBuffers

[msakai]

I encountered following error when I tried to compile Menoh on Travis-CI Trusty environment that uses protobuf-compiler_2.5.0-9ubuntu1_amd64.deb.

...
-- Found Protobuf: /usr/lib/x86_64-linux-gnu/libprotobuf.so;-lpthread;-lpthread (found version "2.5.0") 
onnx/onnx.proto:393:5: Expected "required", "optional", or "repeated".
onnx/onnx.proto:393:17: Missing field number.
onnx/onnx.proto:443:3: Expected "required", "optional", or "repeated".
onnx/onnx.proto:443:15: Missing field number.
...
/home/travis/build/msakai/menoh/menoh/onnx.cpp:15:10: fatal error: external/onnx/onnx/onnx.pb.h: No such file or directory
 #include <external/onnx/onnx/onnx.pb.h>
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [menoh/CMakeFiles/menoh.dir/onnx.cpp.o] Error 1
make[1]: *** [menoh/CMakeFiles/menoh.dir/all] Error 2
make: *** [all] Error 2

It would be nice if required version of ProtocolBuffers is mentioned in README.md

[okdshin]

I checked protoc version 3.5.1 it ok. I'll write it.

[WilliamTambellini]

Could you please add the min needed version directly in menoh CMakeList :
https://cmake.org/cmake/help/v3.1/command/find_package.html
find_package(Protobuf (version) REQUIRED)
?
Tks

[okdshin]

I checked before 2.6.1 is ok. I'll add version check. Thank you @WilliamTambellini !