Increase memory sample rate.

Summary:
The google.appengine.api.runtime API is faster than I thought, so it should be
fine to bump up our sample rate.  This will increase the overhead a bit but
also give more useful data.

Test Plan: Load the homepage with the memory sampling profiler enabled.

Reviewers: alan

Reviewed By: alan

Differential Revision: http://phabricator.khanacademy.org/D10577