Allow get template fixture to accept version
Allow get template fixture to accept version

The other template fixtures do, because the real API call does, it was
omitted from the letter equivalent.

alphagov/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin