Support for core v0.13

[Former user]

#!

diff --git a/lib/gnuplot.ml b/lib/gnuplot.ml
index 2a2fd14..4c114c8 100644
--- a/lib/gnuplot.ml
+++ b/lib/gnuplot.ml
@@ -227,7 +227,7 @@ module Labels = struct
     let cmd =
       [ format_label "xlabel" t.x, "set xlabel"
       ; format_label "ylabel" t.y, "set ylabel"
-      ] |> List.filter ~f:(fun (s, _) -> s <> "")
+      ] |> List.filter ~f:(fun (s, _) -> not (String.equal s ""))
     in
     { Command.
       command = cmd |> List.map ~f:fst |> String.concat ~sep:"\n";
@@ -443,7 +443,7 @@ module Gp = struct
       ] |> List.filter_opt
     in
     List.iter commands ~f:(fun cmd ->
-      if cmd.Command.cleanup <> "" then begin
+      if not (String.equal cmd.Command.cleanup "") then begin
         if t.verbose then printf "Setting:\n%s\n%!" cmd.Command.cleanup;
         send_cmd t cmd.Command.cleanup
       end)