Tweak purcell's config letting it only suit with emacs 24+.

[js2-mode](https://code.google.com/p/js2-mode/): An improved JavaScript
mode for GNU Emacs.

[ac-js2](https://github.com/ScottyB/ac-js2): An attempt at context
sensitive auto-completion for Javascript in Emacs using js2-mode's
parser and Skewer-mode (requires Emacs 24.3).

[coffee-mode](https://github.com/defunkt/coffee-mode): An Emacs major
mode for CoffeeScript and IcedCoffeeScript.

[skewer-mode](https://github.com/skeeto/skewer-mode): Provides live
interaction with JavaScript, CSS, and HTML in a web
browser. Expressions are sent on-the-fly from an editing buffer to be
evaluated in the browser, just like Emacs does with an inferior Lisp
process in Lisp modes. Call `run-skewer` to start server in the brower
to enable its functionalities.