Replace caml_js_const by caml_js_expr

Hello !
Goji is broken with the latest changes in js_of_ocaml, since `caml_js_const` isn't here anymore.
See: https://github.com/ocsigen/js_of_ocaml/commit/36dc6d2593530fcd7a4157743b09b2c722225df3
So I simply replaced it with `caml_js_expr` as suggested by @hhugo on IRC,