Ensure CNAMEs are unique for dnsmasq

dnsmasq will error if you have duplicate CNAMEs in your config:

$ sudo /etc/init.d/dnsmasq restart
 * Restarting DNS forwarder and DHCP server configuration syntax check    [fail]

Remove duplicate CNAMEs using unique().
Merge pull request #173 from alphagov/unique-cnames

Ensure CNAMEs are unique for dnsmasq