update pgocaml dependency to >=3.1 to reflect PPX

opam

@@ -1,6 +1,6 @@
 opam-version: "2.0"
 name: "ocsigen-start"
-version: "1.5.0"
+version: "1.5.ppx"
 authors: "[email protected]"
 maintainer: "[email protected]"
 synopsis: "An Eliom application skeleton ready to use to build your own application with users, (pre)registration, notifications, etc"
@@ -14,7 +14,7 @@ install: [ make "install" ]
 remove: [ make "uninstall" ]
 depends: [
   "ocaml" {>= "4.06.1"}
-  "pgocaml" {>= "2.3"}
+  "pgocaml" {>= "3.1"}
   "macaque" {>= "0.7.4"}
   "safepass" {>= "3.0"}
   "ocsigen-i18n" {>= "3.1.0"}