Parse config file with ERB

Use case: credentials stored in env vars can be included in the config file with ERB.

```yaml
# config/whenever-elasticbeanstalk.yml

staging:
  access_key_id: <%= ENV['AWS_ACCESS_KEY_ID'] %>
  secret_access_key: <%= ENV['AWS_SECRET_KEY'] %>
  region: 'eu-west-1'
production:
  access_key_id: <%= ENV['AWS_ACCESS_KEY_ID'] %>
  secret_access_key: <%= ENV['AWS_SECRET_KEY'] %>
  region: 'eu-west-1'
```