interactive listing sort fixed

When you list availabe remote version with `n ls`, the listing is correctly sorted:

```
    0.8.14
    0.8.15
    0.9.0
    0.9.1
    0.10.0
    0.10.1
```

But when you use the intertive listing to switch node version, the lising is currently like:

```
    0.10.0
    0.10.1
    0.8.14
    0.8.15
    0.9.0
    0.9.1
```

This fix correct this problem and also retrieve only valid node directory under `$VERSIONS_DIR/*` : only directories matching \d+.\d+.\d+ are returned

I'm very happy with this PR