Change to notebook directory when validating (repeat of #880)
Change to notebook directory when validating (repeat of #880)

- This was fixed in #880, but a refactoring to split reading from
  preprocessing has removed the wrapper around the preprocess step.

jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader