activate_this.py: do not duplicate first PATH entry

If the bin dir is already in $PATH, do not prepend it again.
This might happen after `workon` had been used and the virtualenv gets
activated in e.g. Vim again via the Python API.

Ref: https://github.com/pypa/virtualenv/issues/14
Fixes #493

Please note: I am not sure, why virtualenv.py was changed in multiple places, I've just executed `bin/rebuild-script.py`.