abort gracefully if no installed version

When you use n for the first time or if you have removed all the versions under `/usr/local/n/versions` it shows a strange empty message:

```bash

  
    * 

```

This fix just check if there is at least one directory inside `$VERSIONS_DIR`
If no, abort gracefully with the error:

```bash
$ n
  
  Error: no installed version

```