ocamlPackages.camlimages_4_1: fix meta

I should have changed meta.branch in 70e33e3ae34 ("glsurf: use current giflib"),
but I overlooked it.

The bitbucket URL doesn't seem to exist any more, so I've updated the
homepage to the GitLab repository we fetch the sources from.

Fixes: 70e33e3ae34 ("glsurf: use current giflib")

pkgs/development/ocaml-modules/camlimages/4.1.2.nix

···
   '';
 
   meta = with lib; {
-    branch = "4.0";
-    homepage = "https://bitbucket.org/camlspotter/camlimages";
+    branch = "4.1";
+    homepage = "https://gitlab.com/camlspotter/camlimages";
     description = "OCaml image processing library";
     license = licenses.lgpl2;
     maintainers = [ maintainers.vbgl ];