Update docutils to 0.13.1

Python docutils https://pypi.org/project/docutils/0.13.1/ was released on 2016-12-09, while using Release 0.11, released on 2013-07-22.

Release note

http://docutils.sourceforge.net/RELEASE-NOTES.html

Refs.

• Update docutils to 0.14 #4121 (closed)
• cf. #4093 (closed)