v2.1.x branch renamed to v2.x.x

[Arran Cudbard-Bell]

Someone was complaining about the branch name v2.1.x being confusing now were on 2.2.0. There's no point in maintaining a branch for v2.1.x because there's never going to be further development.

To fixup branches in your fork:

git branch -m v2.1.x v2.x.x
git config branch.v2.x.x.merge refs/head/v2.x.x
git push origin v2.x.x
git push origin :v2.1.x

-Arran