Update to work with current Pretix version

[syn7hgg]

Is your feature request related to a problem? Please describe.
Yes. I've tried installing the plugin with the latest Pretix version and it isn't showing up as a payment method in the portal.

Describe the solution you'd like
It would be awesome for the plugin to be updated in order to work with the current version of Pretix.

Describe alternatives you've considered
Using integrated payment methods like PayPal. However, this hasn't worked because our store works with Chilean Pesos, which isn't a supported currency by PayPal.

Thanks in advance! Keep up the awesome work :)