Remove deprecated grunt.util.async / Update devDependencies

* I replaced `grunt.util.async.forEach` with [each-async](https://github.com/sindresorhus/each-async). [grunt.util.async](http://gruntjs.com/api/grunt.util#grunt.util.async) is [deprecated](http://gruntjs.com/blog/2014-03-07-grunt-0.4.3-released).
* I updated [grunt-contrib-jshint](https://github.com/gruntjs/grunt-contrib-jshint) and [grunt-contrib-nodeunit](https://github.com/gruntjs/grunt-contrib-nodeunit).
* I removed [proxyquire](https://github.com/thlorenz/proxyquire) from dependencies. This project doesn't use it.
* I updated the file array of `jshint` task. This repository doesn't include `lib` directory and `test` directory.
* I updated .jshintrc.
  * I removed the global variables of [Mocha](http://visionmedia.github.io/mocha/) because this project doesn't use it.
  * `es5` option is now set per default (http://www.jshint.com/blog/2013-05-07/2-0-0/).
  * From [JSHint v2.5.0](https://github.com/jshint/jshint/releases/tag/2.5.0), `trailing` and `smarttabs` options are removed.
* I updated the image format of the build status badge. SVG badges look beautiful on retina displays.