FIX: PSR-2 codebase. Formatting via phpcbf
FIX: rendering bug in allchanges
FIX: update .gitattributes to not export codecov's config file
FIX: Update SiteTree_versions to the ss4 equivalent SiteTree_Versions
Major versions won't automatically work, and so I've amended the composer requirements
not to allow SS4.
This will also ensure that addons.silverstripe.org correctly reports which modules
work with SS4.