Use DistributedLock in materialized views refresh
Use DistributedLock in materialized views refresh

- Copied from
  https://github.com/alphagov/transition/blob/master/lib/tasks/import/whitehall.rake#L17-L19
  based on advice from Jenny that we should do this so it doesn't run on
  every machine.

alphagov/transition,alphagov/transition,alphagov/transition