Updated OCaml version constraint

Author: mmottl

postgresql.opam

@@ -17,7 +17,7 @@ build: [
 ]
 
 depends: [
-  "ocaml" {>= "4.05"}
+  "ocaml" {>= "4.08"}
   "dune" {build & >= "1.7.0"}
   "base" {build}
   "stdio" {build}