[opam] do not allow atdgen >= 1.13.0
[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

facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer