[ci] Do not use "sudo=false" for travis anymore

See the following blog posts
* https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures
* https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
* https://changelog.travis-ci.com/linux-builds-run-on-vms-by-default-77106

.travis.yml

@@ -21,22 +21,18 @@ matrix:
     - name: "linux - mvn deploy"
       os: linux
       dist: trusty
-      sudo: false
       env: BUILD=deploy
     - name: "linux - build documentation"
       os: linux
       dist: trusty
-      sudo: false
       env: BUILD=doc
     - name: "linux - run sonar"
       os: linux
       dist: trusty
-      sudo: false
       env: BUILD=sonar
     - name: "linux - run coveralls"
       os: linux
       dist: trusty
-      sudo: false
       env: BUILD=coveralls
     - name: "macosx - mvn verify"
       os: osx