dns: tweak regex for IPv6 addresses

The regex used in `dns.setServers()` to match IPv6 addresses in square
brackets uses a capturing group for the port but this info is not
needed.

This commit replaces the capturing group with a non capturing one.

PR-URL: https://github.com/nodejs/node/pull/8665


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>