Update dependencies / Remove mocha-as-promised

* I updated [promise](https://www.npmjs.org/package/promise), [mocha](https://www.npmjs.org/package/mocha) and [fs-extra](https://www.npmjs.org/package/fs-extra).
* I removed [mocha-as-promised](https://www.npmjs.org/package/mocha-as-promised) because [Mocha v 1.18.0](https://github.com/visionmedia/mocha/blob/master/History.md#1180--2014-03-13) supports Promise by default. Actually, [it was deprecated by the owner](https://github.com/domenic/mocha-as-promised/commit/4405c942a261a90772538e551258f2fec6234331).
* I changed the image format of build status badge from PNG to SVG. SVG badges look beautiful on retina displays.