Install fails due to missing dependency "darwin"
I've tried installing nimx with Nim 1.6.12 and 2.0.0, and it errors out in the same place due to a dependency on the package `darwin` which can't be found. Maybe it was removed from the Nimble package repo?

```
...
Downloading https://github.com/yglukhov/plists using git
  Verifying dependencies for plists@0.2.0
    Reading official package list
   Checking for darwin@any version
 Installing darwin@any version
    Reading official package list
    Prompt: darwin not found in any local packages.json, check internet for updated packages? [y/N]
y
    Answer: Downloading official package list
     Trying http://google.com
   Warning: Downloaded packages.json file is invalid, discarding.
     Trying http://google.com/404
   Warning: Could not download: 404 Not Found
     Trying http://irclogs.nim-lang.org/packages.json
     Trying http://nim-lang.org/nimble/packages.json
    Success Package list downloaded.
    Reading official package list
     Error: Package not found.
```

Is there a way to skip this dependency since the name "darwin" indicates something specific to Mac OS X, which I don't need?