Switch to using a specific version of protobuf-c (tag v1.2.1).

(v1.3 or HEAD seem to be broken, didn't attempt to understand why).

(Also, tweak the profile description to highlight Capnet just a little
bit; still need to fixup some demo instructions.)
1 job for master in 14 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #1409
build-ubuntu16

00:00:14