Build: Remove `osx_image: xcode9.4` from `.travis.yml`
Build: Remove `osx_image: xcode9.4` from `.travis.yml`

From https://changelog.travis-ci.com/67738:

 " The Xcode 9.4 image (i.e. `osx_image: xcode9.4`) is now our default
   macOS image (previously it was Xcode 8.3). This means that if you
   aren't specifying the `osx_image:` key in your `.travis.yml` file
   or are using a deprecated image, your builds will now run on the
   Xcode9.4 image. "

sonarwhal/sonar,sonarwhal/sonar,sonarwhal/sonar