Travis: update virtualenv (issue py3.6)

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

00:09:18

passed #150746
step_pythran_then_cython

00:07:46

passed #150745
step_without_pythran

00:02:50