Use map-stream instead of event-stream

* Currently this plugin depends on [event-stream](https://github.com/dominictarr/event-stream) but doesn't use its methods except for [`.map()`](https://github.com/dominictarr/event-stream#map-asyncfunction). The `.map()`
method is actually the same as [map-stream](https://github.com/dominictarr/map-stream). (See these lines. [L12](https://github.com/dominictarr/event-stream/blob/master/index.js#L12) [L22](https://github.com/dominictarr/event-stream/blob/master/index.js#L22))  
  So this plugin doesn't need to load event-stream which includes a lot of unused dependencies. It only has to load map-stream.
* I updated [lodash](https://www.npmjs.org/package/lodash), [chai](https://www.npmjs.org/package/chai), [mocha](https://www.npmjs.org/package/mocha) and [gulp](https://www.npmjs.org/package/gulp).