patch 8.1.0704: building with Ruby 2.6 gives compiler warnings

Problem:    Building with Ruby 2.6 gives compiler warnings.
Solution:   Define a stub for rb_ary_detransient. (Ozaki Kiichi, closes #3779)
diff --git a/src/version.c b/src/version.c
index 0693be1..95f3859 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    704,
+/**/
     703,
 /**/
     702,