add ksh to shells

Got warnings about ksh not being recognized on some boxes:

[2014-02-13 19:11:29] WARN - Can't load wanted shell: ksh. Using default shell.
[2014-02-13 19:11:29] WARN - If you want to help the development of Rex please report this issue in our Github issue tracker.

The commit makes it a bash based one as that seems closest and just works.

Cheers,
Simon