migrater.py: Use datetime.date.today() to find the current date

Instead of hardcoding it in the Makefile.  This reverts the portion of
4c6fa04b (Switching trainee status to enumerated field, 2014-12-15)
that added the TODAY argument.

The .isoformat() call is because I don't know if cohort_start[cohort]
is a string or a datetime.date.  Since we were using string
comparisons before, using a string here seemed safer.