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. From the manual:

> 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
Merge pull request #13 from tomfotherby/master

Make addDocument compatible with Solr client v2.0.0