This reorganizes the creation of `Config.version_defaults` to make it
easier to cherry-pick backported config changes to stable branches:
* Sets `Config[0.4]` as a diff from the previous versioned default, just
  like the other versioned defaults.
* Sets `Config[:current] = Config[VERSION.to_f]`, so it will not need to
  be updated for any x.y.0 release.  Likewise, sets `Config[:next]` to
  `Config[VERSION.to_f + 0.1]` which only rarely needs to be changed.
* Because `Config[:default]` and `Config[:current]` are now derived two
  different ways, both a warning and a test have been added to ensure
  they remain synchronized.
* A few other `Config.version_defaults` tests were added or updated.