(hotfix) fix python version requirement (fix #90)

Fix #90.

Related to: https://github.com/jubatus/jubatus/issues/308 and https://github.com/jubatus/jubatus-python-client/pull/29

Reason why we need to treat this pull-req as hotfix:
* This is an important change to mention in 0.5.0 and we forgot to update the document.