fix travis test run by using tox==1.6.1 version

Due to issue described [here](https://bitbucket.org/hpk42/tox/issue/150/posargs-configerror) talons tox tests won't run with latest release of tox (1.7.1). To keep tests running we must use older version of tox where `{posargs}` replacement works as it should work. This isssue is already resolved but we must wait until new version of tox get released.