afsprefs pane shouldn't hardcode 10.4 sdk

afsprefs pane shouldn't hardcode 10.4 sdk

10.4 sdk isn't available in the release after 10.5 as a default. don't hardcode it
same with afscell

Reviewed-on: http://gerrit.openafs.org/207
Tested-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit a6025e0268fd7cff6ee7de145bdbf59cc350c572)
Change-Id: I404861b1d27c9ff8eef4c61707f14d4c36b7ea9d
Reviewed-on: http://gerrit.openafs.org/941