Loosen the dependency on the logger gem

Resolves dependency conflicts in applications that require
later 1.x-series versions.

-  `~> 1.6`:    don't accidentally select 2.x
-  `>= 1.6.1`:  stay above the previous minimum

Original contribution by @nenadl via simplymeasured@5cd51885d5194444d9c1fb01b85a57c8c80a73bb