getSqlExpression() should not be final

If we need to extend the dialect, we have to overload the getSqlExpression(). See https://github.com/phalcon/cphalcon/issues/2952 and http://forum.phalconphp.com/discussion/1748/date-sub-interval-mysql
Merge pull request #3026 from dugwood/patch-3

getSqlExpression() should not be final