killbill-paypal-express.gemspec: use released version of killbill gem
killbill-paypal-express.gemspec: use released version of killbill gem

Signed-off-by: Pierre-Alexandre Meyer <ff019a5748a52b5641624af88a54a2f0e46a9fb5@ning.com>

daliwei/killbill-paypal-express-plugin,killbill/killbill-paypal-express-plugin,killbill/killbill-paypal-express-plugin,daliwei/killbill-paypal-express-plugin,daliwei/killbill-paypal-express-plugin,killbill/killbill-paypal-express-plugin