.travis.yml: add back "language: c"
.travis.yml: add back "language: c"

of course, protobuf-c uses both C and C++.

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