[requirements.txt] bump protobuf version to 3.4.0
[requirements.txt] bump protobuf version to 3.4.0

googlefonts/fontbakery,graphicore/fontbakery,moyogo/fontbakery,graphicore/fontbakery,moyogo/fontbakery,graphicore/fontbakery,googlefonts/fontbakery,moyogo/fontbakery,googlefonts/fontbakery