dns: use isIp consistently

Currently the DNS module imports isIP from both cares and `net` and
uses both of them both throughout the code base. This PR removes the
direct dependency `dns` has on `net` and uses `isIp` from c-ares all
the time. Note that both functions do the same thing.

PR-URL: https://github.com/nodejs/node/pull/5804


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Conflicts:
	lib/dns.js