Look for api keys in ENV if parse_resource.yml not present

Modified self.load_settings to look for app_id and master_key in environment variables if parse_resource.yml is not present. Accommodates use cases where .yml files are in .gitignore and Heroku's preferred pattern of storing API keys is followed.

Allows for use: 
```heroku config:set app_id=FOOBAR```
```heroku config:set master_key=FOOBAR```

Please note: I'm new to Ruby and this is my first pull request so let me know if I'm not doing this the right way.