Fix detection of existing addons

Looks like something changed in the Heroku gem.

Your detection of existing addons resulted in a list looking like:
```yaml
- 'honeybadger'
- 'development'
- 'papertrial'
- 'choklad'
```
when you needed:
```yaml
- 'honeybadger:development'
- 'papertrail:choklad'
```