🐞 Check the IPv6 (AAAA) record, not the IPv4 (A)

My test had a mistake: I looked for the wrong kind of record. Now it
looks for the right one.

spec/check-dns_spec.rb

@@ -74,7 +74,7 @@ domains.each do |domain|
       end
 
       it "nameserver #{whois_nameserver}'s has an AAAA record" do
-        expect(`#{dig_cmd} @#{whois_nameserver} a #{domain} +short`.chomp).not_to eq('')
+        expect(`#{dig_cmd} @#{whois_nameserver} aaaa #{domain} +short`.chomp).not_to eq('')
         expect($CHILD_STATUS.success?).to be true
       end