Lower publishing-api-sync-checks concurrency in dev
Lower publishing-api-sync-checks concurrency in dev

On my laptop (2013 i7 15" Mac Pro FWIW), inside the vagrant VM,
running a sync check will DOS the local `content-store`, causing
all responses to return a "Bad Gateway" status.

This change lowers the concurrency during development to 1.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall