Bitbucket Pull Request Builder plugin can not connect to bitbucket.org if the Jenkins is behind a proxy.
I attached a patch that solves this bug. This solution uses the http client SystemDefaultHttpClient in the org.apache.httpcomponents:httpclient instead of using the HttpClient in commons-http library. This class use the system properties to configure a proxy as described in https://docs.oracle.com/javase/7/docs/api/java/net/doc-files/net-properties.html.
Another solution is the get the Jenkins' proxy configuration: jenkins.model.Jenkins.getInstance().proxy