[opam-devel] switch CI to use Trusty

[Thomas Gazagnaire]

Hi again,

Since https://github.com/ocaml/opam-repository/pull/6902 we are now using Ubuntu Trusty for the CI of opam-repository. Normally this should be fully transparent but be aware that it might causes some depext errors on the CI which should be fairly easy to fix (as we switched to Trusty to have more up-to-date depexts anyway).

Best,
Thomas