adding JSON response handling

Hi Shiwano,

thank you very much for your grunt-testflight plugin. It is a great help for us. 

We need the download URL for each build, so we added a response handling. We add this URL in our current download page: https://dev.cameo.io/dl/

It would be great if you would add our pull request to your version of your plugin. Currently we published our own version of your plugin. We will remove it from npmjs, if you would accept our pull request. We published it only to be able to use it easier in our project cameoNet: https://github.com/memoConnect

Cheers,
Michael