gemspec: update to Kill Bill 1.0.15

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
api: update to latest Kill Bill API and bug fixes

* Bail in case a payment has already been made for a given kb_payment_id
* Return correct payment and refund responses
* Verify we don't over-refund

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
Merge branch 'master' of github.com:killbill/killbill-paypal-express-plugin