Use seuptools to be explicit about optional version-dependency of markdown.

CHANGELOG.rst

 Release Notes
 =============
 
+0.4.0-dev
+---------
+
+* Markdown < 2.0 is no longer supported.
+
 0.3.3
 -----
 

setup.py

@@ -64,6 +64,9 @@ setup(
     package_data=get_package_data('djangorestframework'),
     test_suite='djangorestframework.runtests.runcoverage.main',
     install_requires=['URLObject>=0.6.0'],
+    extras_require={
+        'markdown': ["Markdown>=2.0"]
+    },
     classifiers=[
         'Development Status :: 4 - Beta',
         'Environment :: Web Environment',