Call transaction before calling rollback_transaction_records

If the protected method rollback_transaction_records is called before
transaction has been called at least once, the
@_current_transaction_records instance variable will be unitialized.
This results is an exception when clean attempts to call
rollback_transaction_records.
Fixed failing active_record/transaction_spec
Merge pull request #191 from cleanoffer/fix_active_record_start_transaction

Call transaction before calling rollback_transaction_records