raise six requirement to 1.4+ when UserString was added

--HG--
branch : techtonik/raise-six-requirement-to-14-when-userstr-1406538263417
Merged in techtonik/sphinx-1/techtonik/raise-six-requirement-to-14-when-userstr-1406538263417 (pull request #259)

raise six requirement to 1.4+ when UserString was added