This reverts commit 189e6f5e.

After some discussion, it seems better not to bump the major version
number since most of the new features since 2.3 are marked
experimental.

Since there are some incompatible changes, it's better to bump the
major version number.

This reverts commit 5a1a8708. Counting
is hard.

Following discussion with Shea and Graham. It's a big enough change
from the last release. Also, from a semver perspective, 2.0 makes more
sense because we did remove some interfaces (like nix-pull/nix-push).

This reverts commit 28bba8c4.

Also use a point release version number as suggested by several
people.