Add the Government Procurement Service as an alias of CCS

- Following a chat with Robin, this is deemed the best way of
  configuring the Government Procurement Service's domain
  gps.cabinetoffice.gov.uk as it appears idential to the new
  ccs.cabinetoffice.gov.uk domain content-wise, and currently,
  pre-transition, it redirects to CCS. The Government Procurement
  Service has closed down.
Merge pull request #1011 from alphagov/configure_gps.cabinetoffice.gov.uk

Add the Government Procurement Service as an alias of CCS