Code refactoring

* I updated dotfiles.
  * I removed unsupported options from `.jshintrc`. [`trailing` and `smarttabs` options are no longer supported](https://github.com/jshint/jshint/releases/tag/2.5.0).
  * I added `sudo: false` to `.travis.yml` [to use container-based infrastructure](http://docs.travis-ci.com/user/workers/container-based-infrastructure/). It's really faster than ever.
* I updated package.json.
  * I added `files` field instead of using `.npmignore` file. white-listing is more consistent than black-listing.
  * I introduced [user/repo style](https://github.com/npm/normalize-package-data/pull/31) to the `repository` field.
* I updated `index.js`.
  * I decrerased nesting depth (keeping your coding style, of course).
  * I removed redundant processes.
    * vinyl-fs can accept node-glob options as its second argument.
* I updated image format of the Travis CI badge. SVG badges look beautiful on retina displays.