Make bin/release comply with published Buildpack API

From https://devcenter.heroku.com/articles/buildpack-api#bin-release:
```
This script returns a YAML formatted hash with two keys:
addons is a list of default addons to install.
default_process_types is a hash of default Procfile entries.
```
This pull request makes the buildpack's bin/release script comply with this specification by having it emit a hash with an empty list for 'addons' and an empty hash for 'default_process_types'.