Add support for multiple nodes, so the :url option could be an array of URLs.

Allows to pass an array of URLs, and in case of `Errno::ECONNREFUSED` RSolr will now try another nodes (if available) before raising the error,
Should this be a public method?  It doesn't look like it to me.  Renaming this "first_or_next_solr_uri" would provide some clarity as to it's intention.