add Package-Requires: cl-lib

[zimoun]

Following the discussion about the dependencies management by package.el and MELPA (see #3855) this patch should fix the issue of the dependency to the function cl-reduce.

• cl-lib is included in Emacs 24.3+, therefore there is no dependency issue anymore about (require 'cl-lib).
• the added line insures that package.el will manage cl-lib, possibly downloading it even for Emacs 24.3+. However, it is small and stable, upgrading barely.

[dacap]

Thanks for taking care about this issue @zimoun :+1: