Drop the concurrency to 2 for sidekiq

The current default of 25 causes an issue due to memory usage where we
cannot create new threads, when doing a whitehall rebuild of the index
see this issue for more information:

https://github.com/mperham/sidekiq/issues/54
Merge pull request #146 from alphagov/reduce-concurrency

Drop the concurrency to 2 for sidekiq