Update dependencies / Remove unused modules

* I updated [async](https://www.npmjs.org/package/async) and [grunt-contrib-jshint](https://www.npmjs.org/package/grunt-contrib-jshint).
* It seems that this plugin does not use [through2](https://www.npmjs.org/package/through2), [cache-file](https://www.npmjs.org/package/cache-file), [stream-combiner](https://www.npmjs.org/package/stream-combiner) and [mocha](https://www.npmjs.org/package/mocha). So I removed them from `dependencies` and `devDependencies`.
* I changed the image format of badges in README from PNG to SVG. SVG badges look beautiful on retina displays.