Support configuration from the Rails secrets.yml file

[nextmat]

The secrets.yml file was added in Rails 4.1 to provide a consistent place for storing sensitive configuration information. It would be cool if for 4.1+ we could mine configuration definitely from that file if a librato key group was present, something like:

production:
  librato:
    user: 'user'
    token: 'xxx'
    ...

This would allow folks on newer rails to have one less config file to manage. Related: #107