Update nbgrader/nbconvert pinned versions in future

[geoff-va]

Had to pin nbgrader/nbconvert in the Dockerfile as something broke in the 0.6.1/6.0.7 combination.

Pinned them to:

• nbgrader==0.6.1
• nbconvert==5.6.1

We also noticed during this error that the assign function will be deprecated in a future release. That will need to be updated to the suggested replacement as well.