requirements: update markdown to the latest version

requirements.txt

 -e git+git://github.com/fredj/cssmin.git@master#egg=cssmin
 Django==2.1.5
 IPy==0.83
-Markdown==2.6.10
+Markdown==2.6.11
 bencode.py==2.0.0
 django-countries==5.0
 jsmin==2.2.2