patch 8.1.0776: Travis does not build a version without GUI on Linux

Problem:    Travis does not build a version without GUI on Linux.
Solution:   Add an environment for tiny features without GUI.
diff --git a/src/version.c b/src/version.c
index 8584fce..da4b2b6 100644
--- a/src/version.c
+++ b/src/version.c
@@ -792,6 +792,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    776,
+/**/
     775,
 /**/
     774,