Update package.json so that Grunt dependencies are not downloaded in production

Hey @cbas! Running `npm install --production` on this module was a little on the heavy side due to Grunt being required in production mode, so I've moved Grunt and dependencies to `devDependencies`. I think RequireJS can also be moved to `devDependencies` too as it only seems to be required in your tests, but I'll wait for your feedback on this before changing it.

Thanks so much for writing this module! FWIW, I will be using it in this project: https://github.com/adamyeats/midgar amongst others :)