Update dependencies and devDependencies

* I updated [gulp-util](https://github.com/gulpjs/gulp-util), [through2](https://github.com/rvagg/through2), [gulp](https://github.com/gulpjs/gulp) and [gulp-jshint](https://github.com/spenceralger/gulp-jshint).
* I updated `.jshintrc`.
  * `trailing` option has been removed from [JSHint v2.5.0](https://github.com/jshint/jshint/releases/tag/2.5.0).
  * `describe` and `it` globals are not necessary because this project doesn't use [Mocha](http://visionmedia.github.io/mocha/).