Bump version to 3.0
Bump version to 3.0

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

NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix