Update memory report to ignore routes that add less than 1MB.

Summary:
It already cut off at 50 for each module, but for the batch modules this didn't
actually do much, and there were still plenty of routes that didn't add any
memory showing up.  Now any route that adds <1MB total won't show up.  (The
data is still logged though.)

Test Plan:
Redirect the emails to myself by editing `_send_email`, and check that I don't
see a long list of routes with added_total 0.00.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: chris, benkomalo, alan, marcia

Differential Revision: http://phabricator.khanacademy.org/D11682