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.