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.

By upgrading `eight-track`, we have effectively removed the fixtures and now Travis CI is calling out to the w3c validator directly. Please update all the HTTP fixtures or revert the version change to `eight-track`.