Add dependency on Emacs 24.3.1

This is the earliest Emacs release coming with `window-state-get` and
`window-state-put`.

eyebrowse.el

@@ -5,7 +5,7 @@
 ;; Author: Vasilij Schneidermann <v.schneidermann@gmail.com>
 ;; URL: https://github.com/wasamasa/eyebrowse
 ;; Version: 0.6.0
-;; Package-Requires: ((dash "2.7.0"))
+;; Package-Requires: ((dash "2.7.0") (emacs "24.3.1"))
 ;; Keywords: convenience
 
 ;; This file is NOT part of GNU Emacs.