Use Arel::Visitors::Oracle12 when Oracle server version is 12c

This pull request supports [Oracle 12c better Top-N query] (http://oracle-base.com/articles/12c/row-limiting-clause-for-top-n-queries-12cr1.php) to Oracle enhanced adapter. 

To make this pull request work Arel needs support `Arel::Visitors::Oracle12`. I'll open another pull request to rails/arel.