Fixes issue with inequality filters

Summary:
Inequality filters are pretty broken. For more details, see: https://code.google.com/p/appengine-mapreduce/issues/detail?id=202

I hit the same error.

Test Plan: Run locally and on znd

Reviewers: mattfaus

Reviewed By: mattfaus

CC: jace, eliana

Differential Revision: http://phabricator.khanacademy.org/D5649