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.

.travis.yml

@@ -7,6 +7,7 @@ python:
     - 3.6
 install:
     - pip install -U setuptools==`grep setuptools versions.cfg | awk '{print $3}'`
+    - pip install six==`grep '^six' versions.cfg | awk '{print $3}'`
     - pip install zc.buildout==`grep '^zc.buildout' versions.cfg | awk '{print $3}'`
     - buildout bootstrap
     - buildout install test alltests