travis: libdabba-rpc needs protobuf packages.

protobuf-c-compiler and libprotobuf-c0-dev are needed to build and link
libdabba-rpc library.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
doc: mention protobuf-c dependencies in README.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>