Release 2.0.0

Summary:
This is basically a bugfix release, plus the addition of `ensure` in
`apt::ppa` and changing the default release in `apt::force` to none. This
latter change is backwards-incompatible, but very minor.

Backwards-Incompatible:
- Change default value of `apt::force` from `'testing'` to `false`

Features:
- Add `ensure` to apt::ppa for absent use case