travis: use xenial instead of trusty

This should also fix an issue mentioned in
https://github.com/systemd/systemd/pull/11196#issuecomment-448123946.

.travis.yml

@@ -1,4 +1,5 @@
 sudo: required
+dist: xenial
 services:
     - docker
 

travis-ci/managers/debian.sh

@@ -39,9 +39,6 @@ for phase in "${PHASES[@]}"; do
             $DOCKER_EXEC apt-get -y update
             $DOCKER_EXEC apt-get -y build-dep systemd
             $DOCKER_EXEC apt-get -y install "${ADDITIONAL_DEPS[@]}"
-            # overlayfs on TravisCI is having trouble delivering inotify events to test-path and test-event.
-            # Let's use tmpfs instead for now.
-            $DOCKER_EXEC mount -t tmpfs tmpfs /tmp
             ;;
         RUN)
             info "Run phase"