Negative reasoning like `allBut` is a bad idea with an open world of
platforms. Concretely, if we add a new, quite different sort of
platform, existing packages with `allBut` will claim they work on it
even though they probably won't.
Travis reports hash mismatch during the darwin build, although the hash has been obtained using 'nix-prefetch-url' (on NixOS). Exclude darwin from platforms until the cause is identified.
