active_merchant: upgrade ActiveMerchant to 1.44.1

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
active_merchant: allow overrides in killbill_spec_helper

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
generators: generate more integration tests

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>