Fixed #17908 -- Made some `contrib.markup` tests be skipped so they don't fail...

Fixed #17908 -- Made some `contrib.markup` tests be skipped so they don't fail on old versions of Markdown. Thanks to Preston Holmes for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17749 bcc190cf-cafb-0310-a4f2-bffc1f526a37