Feature/sonar 6.7+ support

[brianbaker]

The current version of the plugin uses the deprecated sonar.branch parameter. This PR updates that param to use sonar.branch.name. See https://docs.sonarqube.org/display/PLUG/Branch+Plugin

[brianbaker]

I had to remove the sonar.project.branch param entirely as Sonar requires that the first scan of a project not supply sonar.branch.name. After that first scan, sonar.branch.name can be supplied. My recommendation is to remove the parameter entirely. Its easy enough to supply it in the additional args. Thoughts?