superstatic CLI config file format not compatible with firebase

[ithinkihaveacat]

firebase serve can handle a firebase.json that looks like this:

{
  "hosting": {
    "public": "public",
    ...
  }
}

superstatic wants the "parent" hosting key removed, so it looks like this:

{
  "public": "public",
  ...
}

(This should be fixed by #226.)