js-of-ocaml release 6.4.0-2 for unstable

[dgit please-upload source=js-of-ocaml version=6.4.0-2]
[dgit distro=debian split --quilt=gbp]
[dgit upstream-tag=upstream/6.4.0 upstream=f7c6bad679d7427e705575a58d3ced950c3d58d2]