_cleanDirs() {
    let labels = Object.keys(this.dirs);

    let label, directory;
    for (let i = 0; i < labels.length; i++) {
      label = labels[i];
      directory = this.dirs[label];
      if (!fs.existsSync(directory)) {
        this._conf.delete(label);
      }
    }
  }
The `_cleanDirs` method deals with sync issues between the
Configstore and what exists on disk. Non-existent directories
are removed from `this.dirs`.

@private
@method _cleanDirs