patch 8.1.1503: sound test fails on Travis

Problem:    Sound test fails on Travis.
Solution:   Set AUDIODEV to "null".
diff --git a/src/version.c b/src/version.c
index 5a69bef..4d77a57 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1503,
+/**/
     1502,
 /**/
     1501,