Update docutils to 0.15.2

[pyup-bot]

This PR updates docutils from 0.14 to 0.15.2.

Changelog

0.15

============

.. Note::

Docutils 0.14.x is the last version supporting Python 2.4, 2.5,
3.1, and 3.2.

Docutils 0.15.x is compatible with Python versions 2.6, 2.7 and 3.3 to 3.5
(cf. `Python 3 compatibility`_).

* reStructuredText:

- Allow embedded colons in field list field names (before, tokens like
 ``:this:example:`` were considered ordinary text).

- Fixed a bug with the "trim" options of the "unicode" directive.

* languages: Added Korean (ko) mappings and latin.

* Several fixes to keep mor information on source in parsed elements,
isolate documents roles from other documents parsed, smartquotes, 
table gets width and latex table multicolumn cells, ...

Links

• PyPI: https://pypi.org/project/docutils
• Changelog: https://pyup.io/changelogs/docutils/
• Homepage: http://docutils.sourceforge.net/

[pyup-bot]

Closing this in favor of #951