api: update to latest Killbill api

This also fixes a typo in setDefaultPaymentMethod when retrieving the
payment method.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
api: fix addPaymentMethod call in ZuoraPaymentPluginApi

The mapping kb <-> Zuora wasn't stored via ZuoraPluginDao.
This also adds a few fast tests for payment methods.

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