fix irminsule dependency (sha1 -> sha)

spotted by @AltGr, who is also improving the opamfu error message
upon encountering such incorrect dependencies
minor release of core
Merge pull request #2002 from avsm/master

fix irminsule dependency (sha1 -> sha)
Merge pull request #2003 from janestreet/master

minor release of core
fix auto-conflict and auto-depopts of lwt
Merge pull request #2004 from samoht/master

fix auto-conflict and auto-depopts of lwt
Merge branch 'master' of git://github.com/ocaml/opam-repository into camlp4-deps