visionmedia/n — pull request #136 by jeromedecoster (2013-07-09 15:45:25+00:00)
Title: n prev fix

`n prev` fails if you have uninstalled the previous version

If you call n prev and the previous version was uninstalled, the activate_previous function display a success, but it's false

This fix abort with an error message:
```bash
$ n prev

Error: previous version 0.10.12 not installed

```