New package deriving-0.5

With the agrement of the orginal deriving authors [1], we renamed our
fork 'deriving-ocsigen' into 'deriving'. We may keep old packages in
'opam-repository' until we update dependent packages,
i.e. js_of_ocaml/eliom, and while [2] isn't fixed.

[1] https://github.com/ocsigen/deriving/issues/5
[2] https://github.com/OCamlPro/opam/issues/740