:construction: Update Heroku schema 

Hey @bgentry! 
I checked out the repo, `curl`'d the latest Heroku schema, and ran `gen/gen.rb`, but ran into some errors:

```console
$ gen git:schema_update ❯ ./gen.rb 
Loading schema...
Generating account-feature...
Generating account...
Generating addon-service...
Generating addon...
Generating app-feature...
Generating app-setup...
./gen.rb:416:in `resolve_all_propdefs': undefined method `[]' for nil:NilClass (NoMethodError)
	from ./gen.rb:478:in `block in func_arg_comments_from_model_and_link'
	from ./gen.rb:473:in `each'
	from ./gen.rb:473:in `func_arg_comments_from_model_and_link'
	from (erubis:47:in `block in result'
	from (erubis:30:in `each'
	from (erubis:30:in `result'
	from /Users/clint/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/erubis-2.7.0/lib/erubis/evaluator.rb:65:in `eval'
	from /Users/clint/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/erubis-2.7.0/lib/erubis/evaluator.rb:65:in `result'
	from ./gen.rb:555:in `generate_model'
	from ./gen.rb:585:in `block in <main>'
	from ./gen.rb:580:in `each'
	from ./gen.rb:580:in `<main>'
```

not being familiar with the project too much, I'm not sure if this is due to an issue with the schema, or `gen.rb`. 

I started looking into it with some debugging, but haven't gotten far, so I thought I'd start this with hopes you can easily see what I can't, or at least provide some debugging tips.