Updates django_manage so it populates VIRTUAL_ENV

Virtualenv's activate script sets the VIRTUAL_ENV environment variable to the path of the virtualenv. Checking this variable is a reasonably common way to verify that execution is happening in a virtualenv. It would be convenient if this module's virtualenv handling set this environment variable.
Merge pull request #7808 from joelcrocker/patch-1

Updates django_manage so it populates VIRTUAL_ENV