Require a minimum version for six.
Require a minimum version for six.

This avoids an issue where old versions of six [played games with base
classes](https://bitbucket.org/gutworth/six/issue/66/replace-the-implementation-of),
which causes trouble in defining `protorpc.messages.Enum`.

craigcitro/protorpc,claimsmall/protorpc,google/protorpc,claimsmall/protorpc,google/protorpc,google/protorpc,claimsmall/protorpc,claimsmall/protorpc,craigcitro/protorpc,google/protorpc,craigcitro/protorpc,craigcitro/protorpc