update(/*label, type, manifest, links*/) {
    return this.create.apply(this, arguments);
  }
The `update` method aliases the `create` method.

@method update
@param {String} label The label for the cache.
@param {String} type The type of package cache.
@param {String} manifest The contents of the manifest file for the cache.
@param {Array} links Packages to elide for install and link.
@return {String} The directory on disk which contains the cache.