Make sure six is up to date for dependencies

.travis.yml

@@ -58,6 +58,7 @@ matrix:
 # command to install dependencies
 install:
   - "pip install --upgrade setuptools"
+  - "pip install --upgrade six"
   - "pip install cherrypy${CHERRYPY_VERSION} --no-use-wheel"
   - "pip install coveralls pyyaml"
   - "if [[ $TRAVIS_PYTHON_VERSION = 2.6 ]]; then pip install unittest2; fi"