Use latest compiler trunk packages

[benmandrew]

Issue from #28 (comment).

The webhook into opam-repository is now working, but this does not necessarily update when the compiler trunk updates.

It is trickier with the first one as development on the compiler repo happens independently of the opam-repo.
AFAICS, the following situations can arise:

• changes on both repos trigger a cache flush (yay!)
• changes on neither repos retains a cache entry (fine)
• changes on opam-repo, but no compiler changes triggers a cache flush (needless, but harmless)
• changes on compiler repo, but no compiler changes retains a cache entry (we temporarily test a previous version)

This could be fixed by having another webhook into ocaml/ocaml.