As of an internal change to Bitbucket, this resource does not work any more. We originally used mmb/concourse-bitbucket-pullrequest-resource but it looks like yours is better maintained and has the check script already use the API.

The problem itself is described in detail here:
mmb#19

I already prepared a fix for that and will provide an PR for it. Still I am seeking for code review/advice, as I am not sure if the changes I´ve made are without problems.