Should escape grep in nvm_ls

Without the \grep escaping I kept getting output like this:

```bash
$ nvm ls
   (standard
input):2:v0.10.29
```
and
```bash
nvm use 0.10
(standard input):1:v0.10.29 version is not installed yet
```

Because I have `grep` to a crazy alias.

Strange thing is that tests worked in both cases... But this fixed the problem of `nvm ls` and `nvm use` for me.