Update devDependencies / Use SVG badges

* I updated devDependencies.
  * [grunt-mocha-test](https://github.com/pghalliday/grunt-mocha-test) now uses the latest version of [mocha](https://www.npmjs.org/package/mocha), so we don't need to add `mocha` module to `devDependencies` explicitly.
* I changed the image format of the badges from PNG to SVG. SVG badges look beautiful on retina displays.