fix(travis): rolling back to virtualenv
fix(travis): rolling back to virtualenv

It seems that running venv inside virtualenv is a nogo.
I'm rolling the set-up back to the original code as it was working
fine, and travis will keep imposing these virtualenvs.

oesteban/niworkflows,poldracklab/niworkflows,oesteban/niworkflows,oesteban/niworkflows,poldracklab/niworkflows