Bump dependencies for for ELPA installations

When installing this package from [MELPA](http://melpa.milkbox.net/) (or any other ELPA repository in which it might be found) then `cl-lib` should also be installed. This commit adds the corresponding Package-Requires header to install it. Feel free to adjust the required version if desired. And Add dependency on Emacs 24 for using `lexical-binding`.