nginx configuration option to force https

I know that this was discussed in https://github.com/tonycoco/heroku-buildpack-ember-cli/issues/10, but I wanted to bring it up again for discussion.

This PR makes it easy to configuration a HTTP -> HTTPS redirect by setting the `FORCE_HTTPS` environment variable to true. Because this should be encouraged for most Ember applications that use some form of authentication to a backend API, I think it deserves to make it into the project directly. Any production deployment sending passwords or auth tokens should not ship without this.

Implementing it locally can also be a slight head-scratcher because you typically do the redirect this way when you have access to the `port` and `server_name`:

```nginx
server {
    listen 80; 
    server_name example.com www.example.com;
    return 301 https://example.com$request_uri;
}

server {
    listen 443 ssl;
    ...
}
```

Thanks for taking a look!