Use redis.yml values to configure Sidekiq

Bringing Rummager into line with other applications a redis.yml file is
now being used to hold the Redis config values. These are now being used
by Sidekiq.

https://www.pivotaltracker.com/story/show/82640998
Sidekiq implements it...
https://github.com/mperham/sidekiq/blob/master/lib/sidekiq/core_ext.rb