Updated the download URL to download.elasticsearch.org

Because GitHub [discontinued](https://github.com/blog/1302-goodbye-uploads)
their "Downloads" feature, new Elasticsearch releases and other artifacts
will be available from http://download.elasticsearch.org.

This patch changes the download URL to work with the new
[Elasticsearch.org download service](http://www.elasticsearch.org/blog/2012/12/17/new-download-service.html).