(#885) Fix for cloudant error when writing a new checkpoint.

When pouchdb went to write a new checkpoint to cloudant, it was accidentally getting the local db's _rev and trying to insert a new doc with that _rev. Cloundant servers had an issue with that. Please see SO question here for cloudant support's specific response to that bug.
http://stackoverflow.com/questions/17979689/cloudant-case-clause-error-with-pouchdb-when-replicating/17980994