travis: mount tmpfs on /tmp before running the tests

To judge from https://api.travis-ci.org/v3/job/465547774/log.txt,
overlayfs on Travis CI is having trouble delivering inotify events,
which is why `test-path` and `test-event` are failing there.

travis-ci/managers/debian.sh

@@ -39,6 +39,9 @@ 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"