Fix naming

This PR

* Deals with the virtualenv & pip issues that are [fixed in a PR on the python cookbook](https://github.com/poise/python/pull/72) that hasn't been merged into the official yet
* Fixes a naming issue
  * Setting `node[:ipynb][:linux_user]` to something other than ipynb did not result in updating the install path for the virtualenv, etc.
* Pins dependencies for packages