Support Oracle 12c top-N query

This pull requests supports  [Oracle 12c better Top-N query] (http://oracle-base.com/articles/12c/row-limiting-clause-for-top-n-queries-12cr1.php). 

Since this new syntax, i.e  `FETCH FIRST n ROWS ONLY` are supported from Oracle 12.1. we need two separate Arel visitors, `Arel::Visitors::Oracle12` for 12c or higher and `Arel::Visitors::Oracle` for 11g or lower.

Refer [another pull request](https://github.com/rsim/oracle-enhanced/pull/518) to visit `Arel::Visitors::Oracle12` from Oracle enhanced adapter