Fix sweeper spec

Require ActiveSupport extension from audited/active_support
Add security when loading Railtie
Remove Ruby lint warning
Allow to deactivate the use of column version

via config use_version_number
For db performance reason.

Each audit created trigger the following request to calculate version number :
SELECT MAX("audits"."version") AS max_id FROM "audits"  WHERE "audits"."auditable_id" =
9444 AND "audits"."auditable_type" = 'Coupon'

Use column created_at instead of version.
When version is deactivated following methods change :
 Audit#revision(verison) return nil
 Audit#revisions(from_verison) return []
Merge remote-tracking branch 'ajoulie/master'

Conflicts:
	spec/audited/adapters/active_record/sweeper_spec.rb