Travis: update virtualenv (issue py3.6)

3 jobs for topic/default/release-0.4.6 in 9 minutes and 19 seconds
Status Name Job ID Coverage
  Test
passed step_pythran_cython #150747

00:09:18

passed step_pythran_then_cython #150746

00:07:46

passed step_without_pythran #150745

00:02:50