Support multiple source download

* I updated the test to cover various cases. [**It fails**.](https://travis-ci.org/kevva/bin-wrapper/builds/45200870)
  * When the constructor is called without `new`.
  * When a user try to download more than one file.
* I updated [download](https://github.com/kevva/download) module to [my fork](https://github.com/shinnn/download/tree/d5f7c870240459e818d4b19633ab1e9535c4609f).
  * Currently the original `download` module doesn't support multiple source download (kevva/download#43). It causes [jpegtran-bin](https://github.com/imagemin/jpegtran-bin) users a serious problem (imagemin/jpegtran-bin#54). Windows users needs both `.exe` and `.dll` but bin-wrapper downloads only the `.dll` file.
  * If kevva/download#43 is merged, I'll update [the last commit](https://github.com/shinnn/bin-wrapper/commit/a4f3f484c113dd84ea5286e7ca5bb5af858ff7c9).