Add Package-Requires for using lexical-binding

`lexical-binding` is introduced from Emacs 24. The package code "emacs" need to tell user a minimal required version of Emacs.

Quoting [Library-Headers](http://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html):
> The package code automatically defines a package named ‘emacs’ with the version number of the currently running Emacs. This can be used to require a minimal version of Emacs for a package.