killbill_logger: allow user to tweak the log level

In order to turn on DEBUG, the user has to set the log level
(logger.log_level = Logger::DEBUG) as well as make sure Kill Bill
logging is configured in DEBUG mode (e.g. logback.xml).

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
version: bump to 1.7.1

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>