Travis CI: use existing virtualenv
Travis CI: use existing virtualenv

The old way of running Travis CI escaped the Travis virtualenv and
ended up using the default system Python, which is always CPython 2.7.

jhuttner/dx-toolkit,olegnev/dx-toolkit,jhuttner/dx-toolkit,johnwallace123/dx-toolkit,andyshinn/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,andyshinn/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,olegnev/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,dnanexus/dx-toolkit,andyshinn/dx-toolkit,jhuttner/dx-toolkit,andyshinn/dx-toolkit,olegnev/dx-toolkit,andyshinn/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,andyshinn/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,olegnev/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,olegnev/dx-toolkit,olegnev/dx-toolkit,andyshinn/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,olegnev/dx-toolkit,olegnev/dx-toolkit,jhuttner/dx-toolkit,andyshinn/dx-toolkit