added missing newline in `box remove version listing`

The output of Errors::BoxRemoveMultiVersion is now like
the output of Errors::BoxRemoveVersionNotFound. I think
that's expected.

example output without this patch:

```
$ vagrant box remove berendt/ubuntu-14.04-amd64
You requested to remove the box 'berendt/ubuntu-14.04-amd64' with provider
'virtualbox'. This box has multiple versions. You must
explicitly specify which version you want to remove with
the `--box-version` flag.

Versions:  * 0.2.11
 * 0.2.12
 * 0.2.14
```

example output with this patch:

```
$ vagrant box remove berendt/ubuntu-14.04-amd64
You requested to remove the box 'berendt/ubuntu-14.04-amd64' with provider
'virtualbox'. This box has multiple versions. You must
explicitly specify which version you want to remove with
the `--box-version` flag. The available versions for this
box are:

 * 0.2.11
 * 0.2.12
 * 0.2.14
```