lock reason version to 1.4.0

Summary:
This is a temporary measure to avoid a breaking syntax upgrade in reason
(`private` renamed to `pri`).

Fixes #577

Reviewed By: yunxing

Differential Revision: D4543094

fbshipit-source-id: 53e61e8

opam

@@ -34,7 +34,8 @@ depends: [
   "javalib" {>="2.3.3"}
   "ocamlfind" {build}
   "ounit" {="2.0.0"}
-  "reason" {>="1.4.0"}
+  "ppx_deriving" {>="4.1"}
+  "reason" {="1.4.0"}
   "sawja" {>="1.5.2"}
   "xmlm" {>="1.2.0"}
 ]