Updating App.json

Hey, the 'Deploy to Heroku' button on:

http://heroku.github.io/nibs/

is pointing to the non-existent repo http://github.com/heroku/nibs-NEXT

Feel free to reject this PR as the actual app.json changes are just cosmetic stuff I threw together from the button maker - https://www.expeditedssl.com/heroku-button-maker - when I was trying to figure out what was broken.