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 to the logger (especially since it can be inspecting an
  ActiveRecord object if you send one in and it gets serialized/deserialized with GlobalID). 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...">`

Note that there may be a better option for this, like running arguments through the filter_parameters list, but I didn't want to overcomplicate this logging code.