Your PR analysis is attempting (and failing) to use the default server: localhost:9000.
In another circumstance, you would update settings.xml to specify the location but since you're using Travis, you'll pass it on the command line instead: mvn sonar:sonar -Dsonar.host.url=https://sonarcloud.io
BTW, this is working in your normal Travis build because the SonarQube integration for Travis is specifically a Travis-SonarCloud integration, so server is filled in automatically.
