Remove recommendation to add `passenv=HOME` to tox.ini
Remove recommendation to add `passenv=HOME` to tox.ini

I believe `passenv=HOME` is no (or no longer?) necessary, as pipenv will
use the virtualenv it is running in. In addition this adds a
recommendation to add `--ignore-pipfile` to `pipenv install`, to protect
the locked dependency set from accidental changes.

kennethreitz/pipenv