More robust search for django-admin.py

The apply_djang_patches script called from ci_before_script.sh started
failing in the Travis CI build.  The reason was that django-admin.py
wasn't being found.  Try executing the script directly, instead of being
called with ``bash`` from ci_before_script.sh and try explicitly activating
the virtualenv from ``apply_django_patches``.

Addresses #626