Fix pip install of tox in travis.
Fix pip install of tox in travis.

ByteInternet/vcrpy,kevin1024/vcrpy,gwillem/vcrpy,bcen/vcrpy,IvanMalison/vcrpy,poussik/vcrpy,agriffis/vcrpy,yarikoptic/vcrpy,kevin1024/vcrpy,ByteInternet/vcrpy,graingert/vcrpy,poussik/vcrpy,graingert/vcrpy,aclevy/vcrpy,mgeisler/vcrpy