Fixed #18041 -- Removed support for Markdown versions < 2.1, following the 1.5...

Fixed #18041 -- Removed support for Markdown versions < 2.1, following the 1.5 deprecation timeline. Thanks Ramiro Morales for the review.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@17909 bcc190cf-cafb-0310-a4f2-bffc1f526a37