[opam] do not allow atdgen >= 1.13.0

Summary:
We normally use 1.12.0 but this wasn't reflected in our opam file.

Fixes #972

Reviewed By: martinoluca

Differential Revision: D9239614

fbshipit-source-id: 17613047b

opam

@@ -28,7 +28,7 @@ ocaml-version: [ >= "4.04.2" ]
 depends: [
   "ANSITerminal" {>="0.7"}
   "apron"
-  "atdgen" {>="1.6.0"}
+  "atdgen" {>="1.6.0" & <"1.13.0"}
   "base64"
   "cmdliner" {>="1.0.0"}
   "core"