Use .utcnow() rather than .now()

[jhamrick]

Fixes part of #504

@lgpage would you mind checking this PR out and testing whether it fixes that test failure for you?

[lgpage]

$ py.test -x -vv nbgrader/tests/api/ PASSED
$ py.test -x -vv nbgrader/tests/apps/ PASSED
$ py.test -x -vv nbgrader/tests/preprocessors/PASSED
$ py.test -x -vv nbgrader/tests/utils/ PASSED
$ py.test -x -vv nbgrader/tests/formgrader/ PASSED (tested without Jupyterhub)
$ py.test -x -vv nbgrader/tests/nbextensions/ PASSED

[jhamrick]

Woohoo, thanks!

[minrk]

A cool thing I learned recently: if you can rely on Python ≥ 3.2, you can do timezone-aware UTC timestamps with the stdlib:

datetime.datetime.now(datetime.timezone.utc)