Bump version to 1.13.0

Changes since 1.12.0:
  * Syscall.CallAsync improvements (#180, #124, #183).
  * Fix maktaba#log# error on reload (#187).

addon-info.json

@@ -1,7 +1,7 @@
 {
   "name": "maktaba",
   "description": "Consistent Vimscript",
-  "version": "1.12.0",
+  "version": "1.13.0",
   "author": "Google",
   "repository": {"type": "git", "url": "git://github.com/google/maktaba"}
 }