[1.3.X] Fixed #17908 -- Made some `contrib.markup` tests be skipped so they...

[1.3.X] 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.

Backport of r17749 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@17750 bcc190cf-cafb-0310-a4f2-bffc1f526a37