byobu tries to call sed with flags which don't work with BSD sed;
depending on gnu-sed fixes several commands.
This was reported upstream and marked fixed, but still seems to be the
case in current versions:
https://bugs.launchpad.net/byobu/+bug/882813
Upgrade byobu to version 5.21.
Tested on Lion with clang and llvm from XCode-4.3.3.
ClosesHomebrew/homebrew#13399.
Signed-off-by: Misty De Meo <mistydemeo@gmail.com>