Ensure all TXT and SPF records are quoted

According to PowerDNS, "SPF records need quotes, it is a historical anomaly that TXT records can do without. We're not repeating that with SPF."
	- http://wiki.powerdns.com/trac/ticket/249

Note: If there are any existing SPF records which are not quoted, THEY WILL NOT BE HONOURED by PowerDNS. Since this changes email-deliverability, this change does not attempt to fix this. However, if you make any CHANGES, you must add the quotes from here on in.

Fixes https://github.com/kennethkalmer/powerdns-on-rails/issues/6