update the DNS servers we use on prod-lon

* We were using 217.112.87.147, 217.112.88.10 and 217.112.88.90
* Earlier today, 217.112.87.147 stopped responding to DNS requests
* I'm not sure where 217.112.87.147 came from, but the other 2 are
  operated by poundhost.com, the DC used by rimu
* Rimu's official DNS server for the london DC is 92.48.122.126, so I've
  added that as the primary server we use
* Rimu also recommend adding their official DNS servers in New York and
  Dallas as backups. I think that's a terrible idea as it adds at least
  100ms to any DNS resolution we need to make. So I've ignroed their
  advice and left the 2 poundhost.com servers as backups
* We run a risk that poundhost.com will change their servers and we
  won't know until it's too late. If that happens, hopefully future me
  will find this commit message and understand what's going on

  (╯°□°）╯︵ ┻━┻