handle latest version of nbformat, which adds random string ids

Summary:
https://github.com/jupyter/nbformat/issues/196 does not affect this test, since it just
maintains version behavior for existing notebooks, rather than instantiating new cells. This
diff just changes the assertion check, by explicitly setting the minor version to 5, and by
using a regex to handle the newly inserted random ids.

Test Plan: ran test

Reviewers: yuhan, max

Reviewed By: yuhan

Differential Revision: https://dagster.phacility.com/D5997