Add lwt < 4.0.0 constraint

Author: vasilisp

opam

@@ -48,7 +48,7 @@ depends: [
   "base-threads"
   "react"
   "ssl"
-  "lwt" {>= "3.0.0"}
+  "lwt" {>= "3.0.0" & < "4.0.0"}
   "lwt_ssl"
   "lwt_react"
   "ocamlnet" {>= "4.0.2"}