Make a new release without camlp4

[XVilka]

Camlp4 release for OCaml 4.08 is the last release ever - see the official announcement. Since the project already dropped camlp4 dependency it would be good to make the release for opam as well: http://opam.ocaml.org/packages/usb/

[letoh]

Thanks for the information. Camlp4 has been removed from the dependency list in 1.3.1 release.