Activate the virtualenv only when the activation script exists

I use autoenv and virtualenv, but my virtualenv setup is a bit different than yours. I created a virtualenv for flywheel but it's in `$VIRTUALENV_HOME`, not `./flywheel_env`. Unfortunately, every time I `cd` inside the Flywheel source code, I get an error because `./flywheel_env/bin/activate` doesn't exist.

This change prevents users from getting that error if they don't have the same setup as yours.