Build: Use `xcode9.1` image for the macOS builds
Build: Use `xcode9.1` image for the macOS builds

Ref: https://docs.travis-ci.com/user/reference/osx/#OS-X-Version

alrra/sonar,alrra/sonar,sonarwhal/sonar,alrra/sonar,sonarwhal/sonar,sonarwhal/sonar