Fix header for ELPA compatibility

This commit makes `review-mode.el` installable with package.el.

For information about this fix, See following GNU Emacs Manual:

- [Simple-Packages](http://www.gnu.org/software/emacs/manual/html_node/elisp/Simple-Packages.html)
- [Library-Headers](http://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html)