Officially track bourbon via bower

Since WooCommerce utilizes bourbon mixin library for sass which we can officially track the [bourbon](https://github.com/thoughtbot/bourbon) distribution version via bower. Previously [node-bourbon](https://github.com/lacroixdesign/node-bourbon) is used and look its [Changelog](https://github.com/lacroixdesign/node-bourbon/blob/master/CHANGELOG.md) where we can say it didn't officially track bourbon.

### Features ###
* Manages bournbon via bower - `bower.json`
* Git ignore for `bower_components` - `.gitignore`
* Replaced the `loadPath` for sass task options - `Gruntfile.js`
* Updated the npm modules with [npm-check-updates](https://www.npmjs.org/package/npm-check-updates) - `package.json`

### Drawback ###
Previously `npm install` will deal with the installation of all node modules for development.
But now you have to do `npm install && bower install` to additionally install bourbon inside `bower_components`.