Improve test for virtualenv in start-app.sh

Rather than looking for the ``~/.virtualenvs/stagecraft/venv`` directory, it
now looks for ``~/.virtualenvs/stagecraft/venv/bin/activate`` file.

This is to fix the case where virtualenv fails the first time and leaves
a dud directory around (it's happened a few times).
Merge pull request #66 from alphagov/more-robust-start-app

Improve test for virtualenv in start-app.sh