Export DownloadBuilder, so jQueryUI Release Script can use it

Fixes #122.

Depends on:
- https://github.com/jquery/download.jqueryui.com/pull/146

Related changes:
- jQuery UI PR: https://github.com/jquery/jquery-ui/pull/951
- jqueryui.com PR: https://github.com/jquery/jqueryui.com/pull/56