travis: Build protobuf and protobuf-c
travis: Build protobuf and protobuf-c

We need more recent versions than (ancient) Ubuntu 12.04 has.

(Ubuntu has protobuf-c 0.14 or so; protobuf 2.4.0 or so. We want
protobuf-c 1.1.0, which requires protobuf 2.6.0.)

cemeyer/hadoofus,cemeyer/hadoofus