[ppx_trace] Build: Add version limit on ppxlib

Summary:
Version 0.16 of ppxlib breaks merlin in combination with ppx_trace,
due to https://github.com/ocaml-ppx/ppxlib/issues/175.

Reviewed By: jvillard

Differential Revision: D23636202

fbshipit-source-id: 37a5f36b7

sledge/sledge.opam

@@ -25,6 +25,7 @@ depends: [
   "mtime"
   "ppx_compare"
   "ppx_hash"
+  "ppxlib" {< "0.16.0"}
   "shexp"
   "smtlib-utils"
   "yojson"