Change the default dev config to use production

Cheapseats (our testing app) uses production by default, but until
this commit Spotlight was using preview. This causes test failures
because the instance of Spotlight that Cheapseats spins up is using
preview data, but it's making assertions against production
Stagecraft.
Merge pull request #863 from alphagov/default-dev-config-production

Change the default dev config to use production