upgrade PaymentServer
- Prefix all the metric names with "payment_" (https://github.com/PrivateStorageio/PaymentServer/pull/98)
- Enforce a certain number of tokens on redemption (https://github.com/PrivateStorageio/PaymentServer/pull/99)
- Update the README to reflect PaymentServer's new API. (https://github.com/PrivateStorageio/PaymentServer/pull/100)