Install the required six version to make TravisCI happy.

.travis.yml

@@ -7,6 +7,7 @@ python:
     - 3.6
 install:
     - pip install -U setuptools==33.1.1
+    - pip install -U six==$(curl $(awk '$1 ==  "extends" {print $3}' buildout.cfg) 2>/dev/null | awk '$1 == "six" {print $3}')
     - pip install zc.buildout
     - pip install -U coverage coveralls
     - buildout bootstrap

buildout.cfg

@@ -1,6 +1,5 @@
 [buildout]
-extends =
-    https://raw.githubusercontent.com/zopefoundation/Zope/master/versions.cfg
+extends = https://raw.githubusercontent.com/zopefoundation/Zope/master/versions-prod.cfg
 develop = .
 parts =
     interpreter