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
Merge pull request #310 from alphagov/add_redis_config_file

Use redis.yml values to configure Sidekiq

https://www.pivotaltracker.com/story/show/82640998