Update to ZODB Version 4

Forward-port bca6b385 from 2.13 branch.

fix #44 on branch master

Fix two issues

fix cache-invalidation in ProductDispatcher. Fixes all tests that use late-initialized products in test-profiles

Remove Python 2.6 support and skip ipv6 test on Travis

Added note in CHANGES.rst that Python 2.6 is no longer supported.

We skip it when the TRAVIS environment variable is set.
So locally we still run it.

Pep 0440 compatible development version number

Don't bail on WindowsError which is not existing on non-windows systems.

Discussion see https://github.com/zopefoundation/Zope/commit/87c5992e7e40604df206bb713e42bafd5919fe64

support IStreamIterator as WSGI response body

Also, add an assertion that the WSGIResponse gets its 'Content-Length'
header correctly set.

See:  https://bugs.launchpad.net/zope2/+bug/789863

This change applies Leo Rochael's patch from:
https://bugs.launchpad.net/zope2/+bug/789863/+attachment/3624368/+files/0001-Make-the-request-object-unpublishable.patch

And the ideas from Richard Mitchell's patch at:
https://bugs.launchpad.net/zope2/+bug/789863/+attachment/3625765/+files/request_object.patch

Manual cherry-pick of:
81e1fc07
b3f2cf76

[ci skip]
(cherry picked from commit 81e1fc07)

- Add docs for installation into a virtualenv.
- Update Sphinx paths to use '_' prefix.

(cherry picked from commit 6b96bc75)

This reverts commit 367a7ea5.