Required MacOSX version is 10.8

Change-Id: I5dcfdb3106bfe3e83a64443cd8aff1c7ba023a5b
Reviewed-on: https://gerrit.libreoffice.org/12244
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>