tests: add libc-ares to travis dependencies
tests: add libc-ares to travis dependencies

Now that we don't carry it as a submodule, we need to pull it in from the
build environment.
Message-Id: <bff6fa413916cec7d590208fe31c77229ef0e585@scylladb.com>

syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar,syuu1228/seastar,dreamsxin/seastar,dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar