Adopt to upstream changes (use name instead of id for group names)

The pull request [#346](https://github.com/G-Node/nix/pull/346) introduces two changes to the public api:
- Name is read only (for now)
- `xxxPropertyWithName` becomes `xxxPropertyByName`

This build will fail now! Please let travis build it again when [#346](https://github.com/G-Node/nix/pull/346) was merged into master.