v2.3 released

Feature fixed on release 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