Figure out how to handle upgrades of the default repo2docker environment

[moorepants]

Textbook authors will write their books with Jupyter cells. They will get them to work with the current versions of software we have supported in the default-env. But we will want to upgrade default-env with new packages and new package version and even remove packages if we have to. This means we will break existing textbooks on these upgrades. We need to figure out a way so that authors of the textbooks are responsible for keeping their textbooks up-to-date with the upgrades to default-env, not us, as that workload is outside the scope of this team. Maybe we can simply leverage Delmar's team of students that are creating these textbooks to do the upgrades or maybe we need a way for authors to pin their textbooks to specific jupyterhub environment versions.