Update deps / Use SVG badge

* I updated `dependencies` and `devDependencies`.
* I updated the option of the [`jshint` task](https://github.com/gruntjs/grunt-contrib-jshint).
  * Now we don't need to specify the global variables of [Mocha](http://visionmedia.github.io/mocha/) explicitly, because [JSHnit supports the mocha option](https://github.com/jshint/jshint/pull/1597) from v2.5.0.
* I changed the image format of the build status badge from PNG to SVG. SVG badges look beautiful on retina display.