Use new style form to register directives

See
https://github.com/mholt/caddy/commit/17709a7d3f1a6ee0c34bdb81c6332b1339ea7085
where there was a slight change (for the better) on how to register.

Fix the CoreDNS to adhire to this. Needs Caddy from master to compile at
this moment.

core/dnsserver/register.go

@@ -13,7 +13,7 @@ const serverType = "dns"
 
 func init() {
 	caddy.RegisterServerType(serverType, caddy.ServerType{
-		Directives: directives,
+		Directives: func() []string { return directives },
 		DefaultInput: func() caddy.Input {
 			if Port == DefaultPort && Zone != "" {
 				return caddy.CaddyfileInput{