Fix py34 builds

Author: sloria

.travis.yml

@@ -15,6 +15,8 @@ install:
 - travis_retry pip install -U .
 - travis_retry pip install -U -r dev-requirements.txt
 - travis_retry pip install -U apispec"$APISPEC_VERSION"
+# Fix travis issue with py3.4 builds
+- travis_retry pip install -U six
 before_script:
 - flake8 flask_apispec
 script: py.test