always update setuptools before travis run, resolves tox-dev#656

Author: gaborbernat

.travis.yml

@@ -23,7 +23,7 @@ matrix:
  language: generic
 
 before_install: if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then source .travis-osx; fi
-install: pip install --pre -U tox
+install: pip install six && pip install --pre -U tox
 script: tox -e py
 after_success:
  - tox -e coverage