Incompatibility with Sonar 7.7

Starting with Sonarqube 7.7 the preview mode (-Dsonar.analysis.mode=preview) was removed, making it incompatible with the plugin. 

Sonarqube release notes say it now has "native support for short-living branches" https://www.sonarqube.org/sonarqube-7-7/, author will make an effort to integrate these features. Contributions are appreciated 

A place to start with (for getting a json report): https://community.sonarsource.com/t/sonar-report-json-is-this-file-still-available/5827/6

see https://plugins.jenkins.io/sonar-gerrit/#documentation

This is relevant as SonarQube 7.9 is the current LTS version and was released 1. July 2019.