Update Heroku Button url

Hi @atmos,

I've seen there are people who is actually deploying /heaven to Heroku using the Heroku button which is awesome, but the url is pointing out to dashboard-next. We're going to GA soon, and when this happens, this link will be likely broken.

Using `www.heroku.com/deploy` won't break anything, not now, nor when dashboard-next goes GA.

Btw, I proposed the change to `https://heroku.com/deploy` for simplicity, but you could still using `?template` in case you want to make sure forked versions of your repo are still pointing out to yours (`https://heroku.com/deploy?template=https://github.com/atmos/heaven`).

Cheers!