Explicity install six to unbreak Travis.
Explicity install six to unbreak Travis.

dunkhong/grr,dunkhong/grr,google/grr,google/grr,google/grr,dunkhong/grr,dunkhong/grr,dunkhong/grr,dunkhong/grr,google/grr,google/grr,dunkhong/grr,google/grr,google/grr