Vector#flatten: correct behaviour when no argument is given to support Rubinius and JRuby

In case no argument is passed to `Vector#flatten`, we call
`Array#flatten(nil)`; but the correct argument to pass is -1 to indicate
endless recursive flatenning.

While MRI accepts both -1 and `nil`, Rubinius and JRuby don't accept an
explicit `nil` argument to `Array#flatten`.