Clarify and simplify supported versions of Wagtail

[loicteixeira]

At the moment, Wagtail Bakery supports Wagtail >= 1.6. While there are no real issues with supporting such an old version, it probably isn't the best use of resources (having to constantly ensure backward compatibility) and is probably worthless as it is unlikely that Wagtail 1.6 is still being used anywhere.

During the Wagtail core team meeting of September 12th 2018, it was decided that to only support the following versions: LTS version, current and current-1.

At the moment, it would mean 2.2, 2.1, 1.13 and 1.12 but with the next LTS version (2.3) around the corner and the breaking changes of Wagtail 2.0, it was decided that the package would drop compatibility with Wagtail prior to 2.0.

Unless someone makes a strong counter argument, this will be implemented in the next release.

TODO

• Document supported versions of Wagtail
• Update test matrix to test said versions

[loicteixeira]

As of today, it would actually mean 2.2 (current-1) and 2.3 (the LTS and current).