patch 8.1.1503: sound test fails on Travis

Problem:    Sound test fails on Travis.
Solution:   Set AUDIODEV to "null".

.travis.yml

@@ -128,6 +128,8 @@ before_script:
     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:

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1503,
 /**/
     1502,
 /**/