Shut up E_STRICT message

E_STRICT: Declaration of
vierbergenlars\Norch\ODM\SearchIndex::addDocument() should be compatible
with vierbergenlars\Norch\SearchIndex\Index::addDocument($id, array
$document) in /home/travis/build/vierbergenlars/Norch-
PHP-Client/lib/vierbergenlars/Norch/ODM/SearchIndex.php on line 39