use `typeset` instead of `local` in rbenv() function
use `typeset` instead of `local` in rbenv() function

This is to insure portability to ksh.

Fixes #205, references #209

cehoffman/rbenv,cehoffman/rbenv,cehoffman/rbenv,mediadoneright/rbenv,mediadoneright/rbenv,mediadoneright/rbenv