New deploy procedure

- Deploying to staging is a normal push
- Deploying to production releases the current slug from staging

I just used this to (migrate-)deploy latest to production.
Yes, without the shell would try to run the result as a command. It's a common idiom for variable initializing and is equivalent to

```sh
HEROKU="${HEROKU:-https://api.heroku.com}"
```

Note `:-` vs `:=`.