Update deps / Use SVG badge

*  <del>I updated `dependencies` and `devDependencies`.</del>
* <del>I updated the option of the [`jshint` task](https://github.com/gruntjs/grunt-contrib-jshint).</del>
  * <del>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</del>.
* I changed the image format of the build status badge from PNG to SVG. SVG badges look beautiful on retina display.