Add Less to package.json. Fix package.json formatting.

This is necessary as the [`--yui-compress` option](https://github.com/FortAwesome/Font-Awesome/blob/4.3.0-wip/src/Makefile#L30) was deprecated in `Less@1.5`.

I had issues when building with `jekyll` and it took a while to track this down.. I had a global `lessc` executable on version `2.1.1`.