patch 8.1.0568: error message for NUL byte in ScreenLines breaks Travis CI

Problem:    Error message for NUL byte in ScreenLines breaks Travis CI.
Solution:   Use a normal message fornow.
diff --git a/src/version.c b/src/version.c
index c4d9a8f..eeb7cb0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    568,
+/**/
     567,
 /**/
     566,