fix: fixed orgDomain lookup (closes #60)

[titanism]

With the pull request:

> require('./lib/dmarc/get-dmarc-record')('pay-dartford-crossing-fine.service.gov.uk').then(console.log)
Promise {
  <pending>,
  [Symbol(async_id_symbol)]: 588,
  [Symbol(trigger_async_id_symbol)]: 582
}
> {
  v: 'DMARC1',
  p: 'reject',
  sp: 'reject',
  fo: '1',
  rua: 'mailto:dmarc-rua@dmarc.service.gov.uk',
  ruf: 'mailto:dmarc-ruf@dmarc.service.gov.uk',
  rr: 'v=DMARC1; p=reject; sp=reject; fo=1; rua=mailto:dmarc-rua@dmarc.service.gov.uk; ruf=mailto:dmarc-ruf@dmarc.service.gov.uk',
  isOrgRecord: true
}

Without the pull request:

> require('mailauth/lib/dmarc/get-dmarc-record')('pay-dartford-crossing-fine.service.gov.uk').then(console.log)
Promise {
  <pending>,
  [Symbol(async_id_symbol)]: 114,
  [Symbol(trigger_async_id_symbol)]: 108
}
> false

[CLAassistant]

All committers have signed the CLA.

[shaunwarman]

+1 :clap: