Try without pinning the six version.

.travis.yml

@@ -5,7 +5,6 @@ python:
     - 3.5
     - 3.6
 install:
-    - pip install -U six==$(curl $(awk '$1 ==  "extends" {print $3}' buildout.cfg) 2>/dev/null | awk '$1 == "six" {print $3}')
     - pip install coverage coveralls
     - python bootstrap.py
     - bin/buildout