Fix thread safety logging issue

closes #19.

It took a few man-days to track down this production issue where logging stops after a few seconds. The problem occurs with a combination of activerecord-session_store, pg and rails_12factor under a threaded web server (puma).

We didn't know specifically where the issue came from, hence:

* Rails: https://github.com/rails/rails/issues/14031
* Puma: https://github.com/puma/puma/issues/465
* lograge: https://github.com/roidrage/lograge/issues/65
* Heroku forums: https://discussion.heroku.com/t/rails-logging-stops-a-few-minutes-after-boot-threadsafety/455

An example app to reproduce this issue is here:
https://github.com/GetJobber/logging_example

Is there an alternative to `logger.quietly` that is thread-safe or should we just remove it?