litle: support wiredump_device

By specifying a wiredump device, such as:

  ActiveMerchant::Billing::LitleGateway.wiredump_device = File.open('/var/tmp/litle.log', 'w')

the device will now be properly passed to the LitleOnline gem for logging.

The LitleOnline gem uses its own connection mechanism and the logging hook will be available
only in versions more recent than 8.15.

Testing Done: tested the integration with the Kill Bill Litle plugin (see http://kill-bill.org).

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>