Fix scripts to behave correctly when run in existing virtualenv

If VIRTUAL_ENV environment variable is set, don't even try to activate
our own.

This may cause unexpected behavior if an unrelated virtualenv is
active, but developers that use virtualenv in other projects should be
careful about that anyway.
Add fab task for updating dependencies
Merge pull request #1427 from learning-unlimited/check-existing-virtualenv

Fix scripts to behave correctly when run in existing virtualenv