Refactor: change variable name in Rakefile

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Bugfix: 2nd argument of uptodate? should be array

Ruby 1.9 won't allow non-array old_list argument for FileUtil.uptodate?.
Which raise error when run rake task under ruby 1.9.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>