Make logjam agent work on heroku.

Hi Stefan,

on Heroku your app is not allowed to write log files to the file system. Instead it should write its logs to STDERR. I've just copied the behaviour of Rails:

https://github.com/rails/rails/blob/08754f12e65a9ec79633a605e986d0f1ffa4b251/railties/lib/rails/application/bootstrap.rb#L46

Cheers,
plu