Hide ActiveJob arguments

* Changes the default ActiveJob logger to log arguments at the debug
  level, since inspecting all arguments has the potential for exposing
  sensitive information (especially since it can be inspecting an
  ActiveRecord object). Rails doesn't normally log potentially
  sensitive, unfiltered arguments above the debug level.

Example output from ActiveJob's logger:

`[ActiveJob] Enqueued UserUpdateJob (Job ID: d6f1040b-fb28-448b-8e3f-ff7ebb35fb5f) to Sidekiq(default) with arguments: #<User id: 1, created_at: "2014-10-27 00:40:08", updated_at: "2014-11-24 14:56:30", email: "example@example.com", password_digest: "$2a$10$1234...">`