Raise minimum ocaml version to 4.08 - which is also required for ppxlib

Author: andersfugmann

ppx_protocol_conv.opam

@@ -11,8 +11,8 @@ build: [
   ["dune" "runtest" "-p" name "-j" jobs] {with-test}
 ]
 depends: [
-  "ocaml" {>= "4.07"}
-  "base" {>= "v0.14.0" }
+  "ocaml" {>= "4.08"}
+  "base" {>= "v0.14.0"}
   "dune" {>= "1.2"}
   "ppxlib" {>= "0.36.0"}
   "ppx_sexp_conv" {with-test}