Update bundlescript shebangs to be bash, reflecting how they're actually invoked

```
15:45:02  tianon:> they really are run as bash
15:45:05  tianon:> invoked within bash
...
15:46:37  tianon:> they get sourced directly within make.sh
15:46:39  tianon:> which is bash
15:46:46  tianon:> that's why those bash arrays work
15:46:50  tianon:> those definitely aren't posix :)
```

See https://github.com/dotcloud/docker/blob/master/hack/make.sh#L70 (and line one of make.sh, where it's clearly a bash script)