Relax core_unix dependency to re-enable mac support

[AndreasDahl]

core_unix.v0.15.0 is currently not supported on macos because it depends on ocaml_intrinsics.v0.15.0 which currently does not support macos.

It does seem like a fix is underway, but can we please relax the dependency on core_unix (and core i suppose) to at least v0.14.0 while we wait for a fix?

Thank you

[AndreasDahl]

The problem manifests like this:

> opam install aws-s3-async.4.6.0
[ERROR] Package conflict!
  * Missing dependency:
    - ocaml_intrinsics
    unmet availability conditions: '(arch = "x86_32" | arch = "x86_64") & os != "win32" & os != "macos"'

No solution found, exiting

I have created PR #29 that relaxes the dependency. It installs perfectly fine on my mac

[andersfugmann]

Closed by #29