Invoke basename with -a, so that it accepts multiple parameters

`nvm ls` doesn't work on my system, because it passes multiple arguments to `basename`. This commit fixes it.

Before:

```
$ nvm ls
       N/A
current: 	v0.10.5
default -> 0.10 (-> v0.10.5)
```

After:

```
$ nvm ls
   v0.6.21	   v0.8.23	   v0.10.5
current: 	v0.10.5
default -> 0.10 (-> v0.10.5)
```

I'm using Ubuntu 13.04 with basename (GNU coreutils) 8.20 and GNU bash 4.2.45(1)-release. The `-a` option added in this commit works on Mac OS X version 10.8.3 as well, so it should be good.