production: set log level to warn, instead of debug

This way, IP addresses won't be logged, which is reasonable for a
software like schleuder-web. Also, this saves quite some space due to
the reduced logging. Still, if required, people are able to increase the
logging manually if there is a need to, or they prefer more verbose
logging in general.

Closes #115
4 jobs for debian/buster/0.0.2 in 5 minutes and 8 seconds
latest
Status Name Job ID Coverage
  Static
passed codespell #144958

00:00:22

 
  Test
passed ruby:2.4 #144959

00:04:42

passed ruby:2.5 #144960

00:04:42

passed ruby:2.6 #144961

00:04:45