Downgrade commonmark to 0.5.5

In 6.x changelog

> DocParser has been renamed to Parser.

https://github.com/rtfd/CommonMark-py/releases/tag/0.6.0

and that it's not compatible with the latest version of
recommonmark (0.4.0)

Author: humitos

requirements/pip.txt

@@ -68,7 +68,11 @@ djangorestframework-jsonp==1.0.2
 
 # Docs
 sphinxcontrib-httpdomain==1.5.0
-commonmark==0.7.4
+
+# commonmark 0.5.5 is the latest version compatible with our docs, the
+# newer ones make `tox -e docs` to fail
+commonmark==0.5.5
+
 recommonmark==0.4.0
 
 # Version comparison stuff