Increase sidekiq redis timeout

We've been seeing timeouts recently on the healthcheck requests. Traced back to the redis query made by `ScheduledPublishingWorker.queue_size`. This will generally respond instantaneously, but occasionally takes longer than a second to respond (or at least it is since we upgraded redis).

See https://errbit.production.alphagov.co.uk/apps/53020d6c0da11585f10000e7/problems/53e9fbd50da1159146003f49