FileUtils::mktemp fix for OS X 10.6.7 and earlier

FileUtils::fu_get_gid only started doing the conversion of the group
to_s automatically from OS X 10.6.8 (ruby-1.8.7-p358) forward.

OS X 10.6.7 (ruby-1.8.7-p174) would fail in brew's FileUtils::mktemp with the
error "Error: can't convert Fixnum into String."

Fixes Homebrew/homebrew#49045
Fixes Homebrew/homebrew#49348

Closes Homebrew/homebrew#49369.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>