tmux 1.6: fix capture-pane test

I'm back on a machine with tmux 1.6. With the current master, 

```
$ sh sh/tmuxwords.sh
tmux: unknown option -- p
usage: capture-pane [-b buffer-index] [-E end-line] [-S start-line] [-t target-pane]
```

It seems odd, because this shows nothing at the command line:

```
$ if tmux capture-pane -p &> /dev/null; then echo 'hi' ; fi
```

Anyways, the issue is avoided by wrapping the condition in `[[ ]]`.