Update README.md for log-level

The default example for setting log level is `config.log_level = :info`... not `config.logger.level = Logger::INFO`. This lead to some confusion during setup when my production environment switched from INFO (which it was set to with config.log_level) to debug whenever I used this papertrail gem.

This may be responsible for the surprisingly large number of production environments in debug you guys have encountered...

> *Confirm existing verbosity*: make sure your app is not unintentionally logging at DEBUG level. **This is much more common than it sounds.**

([Quote from papertrail's support page](http://help.papertrailapp.com/kb/configuration/controlling-verbosity/))