Added our own (.SE's) asnlookup-domain as the new default with Cymru as backup

The domain "asnlookup.iis.se" is now stable enough to be the default instead of Cymru, this is also closing issue 95 (https://github.com/dotse/dnscheck/issues/95). This should also mean that ASN-lookups, at least within Europe, should be completed faster (from a random place in Sweden, 30ms instead of 120ms).