Use Package-Version instead of Version

[CI breaks](https://travis-ci.org/clojure-emacs/clojure-mode/jobs/295224841) without this? Maybe a change introduced in Emacs 25.2?

Author: xiongtx

clojure-mode-extra-font-locking.el

@@ -4,7 +4,7 @@
 ;;
 ;; Author: Bozhidar Batsov <bozhidar@batsov.com>
 ;; URL: http://github.com/clojure-emacs/clojure-mode
-;; Version: 3.0.0
+;; Package-Version: 3.0.0
 ;; Keywords: languages, lisp
 ;; Package-Requires: ((clojure-mode "3.0"))
 

clojure-mode.el

@@ -10,7 +10,7 @@
 ;;       Artur Malabarba <bruce.connor.am@gmail.com>
 ;; URL: http://github.com/clojure-emacs/clojure-mode
 ;; Keywords: languages clojure clojurescript lisp
-;; Version: 5.7.0-snapshot
+;; Package-Version: 5.7.0-snapshot
 ;; Package-Requires: ((emacs "24.4"))
 
 ;; This file is not part of GNU Emacs.