ask report_importer.py to only copy the most recent day of daily_exercise_stats to mongo

Summary: I have no idea why this wasn't set up to do incremental copy in the first place, which makes me a little nervous.

Test Plan: fingers crossed.  we'll see tomorrow.

Reviewers: eliana, robert

Reviewed By: robert

Differential Revision: http://phabricator.khanacademy.org/D2334