#141 change delayed_job.handler to mediumtext (16MB) like event.payload

This isn't a complete fix because if the event.payload is (say) exactly
16MB then the handler will be >16MB and the same problem will occur.

I'll push this in for now though, in practice it means it's much less
likely to occur.
Merge pull request #152 from Alex-Ikanow/update_handler_text_size

#141 change delayed_job.handler to mediumtext (16MB) like event.payload