Hotfix unavailability of ocamlgraph.1.8.8 sources

[dra27]

I'm working on a better proposal to propose to ocamlgraph's maintainers, but I think should unblock the deployment in the meantime.

[dra27]

Assuming this passes deployability, I propose pushing this commit to live, but not merging the PR - rather than editing the old URLs to point elsewhere, I'm hoping that instead the tarballs can be correctly put in the ocamlgraph repo (i.e. fix the 404 rather than change the URL)

[mtelvers]

Looks good to me. I am running this locally, and it has long passed the failure point.

[dra27]

Upstream suggestion in https://github.com/backtracking/ocamlgraph/issues/140

[dra27]

The alternative will be to do a PR with https://github.com/dra27/opam-source-archives and update the package URLs in opam-repository, but personally I think it's slightly nicer for projects to host their own historical archives where possible 🙂