Holder: SS4 compatibility

[robbieaverill]

Requirements:

• [x] Add namespacing
• [x] Change "code" to "src"
• [x] Update code as required to match new core APIs
• [x] Update tests
• [x] Freeze translations in SS3 branch
• [x] Update translations
• [x] Beta tag released

Pull requests

• [x] https://github.com/silverstripe/silverstripe-akismet/pull/23
• [x] https://github.com/silverstripe/silverstripe-akismet/pull/24

[robbieaverill]

PR up for updating the translations at #24. I've frozen the translations in the SS3 branches.

[robbieaverill]

https://github.com/silverstripe/silverstripe-akismet/releases/tag/4.0.0-beta1