Make addDocument compatible with Solr client v2.0.0

v2.0.0 of the PHP Solr client was released on 2014-06-24. It flipped a flag. 

There's a big pink warning box in [the manual](http://www.php.net/manual/en/solrclient.adddocument.php):

> Warning
> PECL Solr < 2.0 $allowDups was used instead of $overwrite, which does the same functionality with exact opposite bool flag.
> $allowDups = false is the same as $overwrite = true