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.

Ref: #8965
PR-URL: #8665
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Ilkka Myller <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Prince John Wesley <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>

lib/dns.js

@@ -292,7 +292,7 @@ exports.setServers = function(servers) {
     if (ver)
       return newSet.push([ver, serv]);
 
-    var match = serv.match(/\[(.*)\](:\d+)?/);
+    var match = serv.match(/\[(.*)\](?::\d+)?/);
 
     // we have an IPv6 in brackets
     if (match) {