.travis.yml: fetch the protobuf tarball from GitHub instead of Google Code
.travis.yml: fetch the protobuf tarball from GitHub instead of Google Code

Per the protobuf developers,

    We will from everything away from Google Code eventually. We haven't
    decided where to put future release packages yet but as it seems github
    supports this well chances are we'll use github as the canonical location
    for all downloads.

koolhazz/protobuf-c,koolhazz/protobuf-c,hongbinz/protobuf-c,y12uc231/protobuf-c,koolhazz/protobuf-c,hongbinz/protobuf-c,lanniaoershi/protobuf-c,tutu12777/protobuf-c,tutu12777/protobuf-c,virqin/protobuf-c,pepsi7959/protobuf-c,y12uc231/protobuf-c,y12uc231/protobuf-c,virqin/protobuf-c,Sannis/protobuf-c,pepsi7959/protobuf-c,lanniaoershi/protobuf-c,Sannis/protobuf-c,pepsi7959/protobuf-c,lanniaoershi/protobuf-c,hongbinz/protobuf-c,virqin/protobuf-c,tutu12777/protobuf-c,Sannis/protobuf-c