crontabberise drop_old_partitions; bug 1014128; attempt #2

This addresses bug 1014128. Again. (Previous attempt at rebasing was a horrible disaster. :unamused:) 

`?r` @peterbe and/or @rhelmer :revolving_hearts: 
Replace this with `from socorro.unittest.cron.jobs.base import IntegrationTestBase` and on line 16 drop the "Case" part.