Vim: replace useless "jsx" snippet with useful one

I originally set up that snippet when I was doing some in-browser
prototyping (ie. with the client-side JSX transform). Now I am using
either browserify/reactify (to prepare client-side resources) or
grunt/jsx (to prepare server-side ones) and so this snippet is entirely
useless. I'm not using the ".jsx" file extension anywhere.

Replace it with something more useful.

Signed-off-by: Wincent Colaiuta <win@wincent.com>