Developer account for CI pipeline

[florish]

We currently cannot make CI builds pass, as we do not have a Buckaroo developer account for which we can safely put credentials on a CI server.

To be sure: we don't need these credentials for doing actual transaction requests on the CI server, the vcr gem can handle this nicely for us. The reason we need them is that recorded VCR responses have a response signature baked in, which can only be verified with the same BUCKAROO_CLIENT_SECRET that was used when the request was recorded. Recording with VCR only makes sense with a developer account, as we do not want to store a real Buckaroo user's SECRET on a CI server for obvious reasons.

Unfortunately, requesting a developer account is a manual process at Buckaroo, which involves signatures on paper and sending stuff by snail mail, which is why we haven't done this so far.