Avoid docutils 0.15

[dharmendra]

pep8 runtests: commands[1] | doc8 -e .rst doc/source/ CONTRIBUTING.rst HACKING.rst README.rst TESTING.rst
Traceback (most recent call last):
  File ".tox/pep8/bin/doc8", line 6, in <module>
    from doc8.main import main
  File "/home/ubuntu/codeup/tacker/.tox/pep8/local/lib/python2.7/site-packages/doc8/main.py", line 48, in <module>
    from doc8 import checks
  File "/home/ubuntu/codeup/tacker/.tox/pep8/local/lib/python2.7/site-packages/doc8/checks.py", line 21, in <module>
    from docutils import nodes as docutils_nodes
  File "/home/ubuntu/codeup/tacker/.tox/pep8/local/lib/python2.7/site-packages/docutils/nodes.py", line 32, in <module>
    import docutils.utils
  File "/home/ubuntu/codeup/tacker/.tox/pep8/local/lib/python2.7/site-packages/docutils/utils/__init__.py", line 20, in <module>
    from docutils import nodes
ImportError: cannot import name nodes
ERROR: InvocationError: '/home/ubuntu/codeup/tacker/.tox/pep8/bin/doc8 -e .rst doc/source/ CONTRIBUTING.rst HACKING.rst README.rst TESTING.rst'
___________________________________________________________________ summary ___________________________________________________________________
ERROR: pep8: commands failed

[Ben Nemec (bnemec)]

This is also blocking the oslo.policy gate.

[Ben Nemec (bnemec)]

Looks like g-r has moved past the broken version, so this is working again.