Use of deprecated sql_attr_str2ordinal in sphinx.conf

Hi,

starting with Sphinx 2.1.2 the used sql_attr_str2ordinal in:

https://github.com/phpbb/phpbb/blob/develop/phpBB/phpbb/search/fulltext_sphinx.php#L287

and

https://github.com/phpbb/phpbb/blob/develop/phpBB/docs/sphinx.sample.conf#L41

are now deprecated according to:

http://sphinxsearch.com/docs/current.html#conf-sql-attr-str2ordinal

and probably will be removed in future versions of Sphinx. According to the shown message:

WARNING: key 'sql_attr_str2ordinal' is deprecated in /etc/sphinxsearch/sphinx.conf line 39; use 'sql_attr_string for sorting' instead.

it should be replaced by:

http://sphinxsearch.com/docs/current.html#conf-sql-attr-string