Use same version of tox on travis.
Use same version of tox on travis.

StackStorm/st2,Plexxi/st2,StackStorm/st2,Plexxi/st2,StackStorm/st2,nzlosh/st2,nzlosh/st2,Plexxi/st2,nzlosh/st2,StackStorm/st2,Plexxi/st2,nzlosh/st2