Fix callbacks not being created when updating audit options

[chaadow]

The main use case is that audited allows users to update the audited options by calling audited multiple times.

# This create two callbacks `after_update` and `after_destroy`
audited on: [:update, :destroy]

# Now if we call audited a second time
# this unfortunately won't create the `after_create` callback when updating the configuration
audited on: [:update, :destroy, :create]

This pull requests fixes the issue so that it creates callbacks only for added configuration, in the example above it's the create one.

[amkisko]

Hey! Does this happen when you inherit the base class with audited call?