patch 8.1.1503: sound test fails on Travis

Problem:    Sound test fails on Travis.
Solution:   Set AUDIODEV to "null".
diff --git a/.travis.yml b/.travis.yml
index 53331e6..3a01234 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -128,6 +128,8 @@
     if [[ "${TRAVIS_OS_NAME}" = "linux" ]]; then
       export DISPLAY=:99.0
       sh -e /etc/init.d/xvfb start && sleep 3
+      # pretend to play sounds
+      export AUDIODEV=null
     fi
 
 script: