add ksh to shells

Simple patch adding ksh to the shells to silence the following warnings:

[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.

Close enough to bash and so far just works.

Cheers,
Simon