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

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 and the overlay2 filesystem driver

Other:

  • the .gitignore file has been simplified