installation fails on building sail=0.13

[bensimner]

When building rmem from scratch, it pulls sail from our local repos https://github.com/rems-project/opam-repository/tree/opam2/packages/sail

If it pulls sail=0.13 then a problem can happen, because our repo's opam file is underconstrained, and if opam tries to pull omd=2.0, the build will fail.

[db7]

This is still an issue. It took me a while to actually realize the solution would be so simple (#21). Why not merge the PR?