Maintenance release v2.3
Feature fixed on v2.3:
* fix compatibility with dnspython 2.0 (#11)
* fix issue when trying to apply DNS update on secondary server (thanks Sebastian Koechlin)
* fix debug message when CNAME was not found
* fix test chain assertion (a chain contains more than 1 certificate)
Feature added:
* skip already validated authorization challenge
Continuous Integration improvement:
* Debian Stretch tests were using Debian Jessie image
* Enable Docker build uses [BUILDKIT](https://docs.docker.com/develop/develop-images/build_enhancements/)
and the `overlay2` filesystem driver
Other:
* the .gitignore file has been simplified