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>

Library/Homebrew/extend/fileutils.rb

@@ -22,6 +22,8 @@ module FileUtils
     else
       Process.gid
     end
+    # Make OS X 10.6.7 (ruby-1.8.7-p174) and earlier happy.
+    group_id = group_id.to_s
     begin
       chown(nil, group_id, tmp)
     rescue Errno::EPERM