[Checkins] [zopefoundation/Zope] 06c8c9: Install the requested six version

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/zopefoundation/Zope
  Commit: 06c8c99e91c332a675519b2464679c99d6202a1a
      https://github.com/zopefoundation/Zope/commit/06c8c99e91c332a675519b2464679c99d6202a1a
  Author: Michael Howitz <mh at gocept.com>
  Date:   2017-10-09 (Mon, 09 Oct 2017)

  Changed paths:
    M .travis.yml

  Log Message:
  -----------
  Install the requested six version

Otherwise Python 3.6 has an error because it seems to be pre-installed on TravisCI but with another version than needed.