patch 9.0.0522: build fails on Appveyor

Problem:    Build fails on Appveyor.
Solution:   Select Visual Studio 2015 for the build tools.
diff --git a/src/version.c b/src/version.c
index 36d4f99..52e36ab 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    522,
+/**/
     521,
 /**/
     520,