Recover from invalid sequence file

Here's the comment from the CouchRest::Changes commit:

So far if the sequence file contains an invalid value the couch request for the changes feed failed and we retried again and again with the same query.

Now we will reset the since parameter to nil instead and thus restart from scratch and process all documents again. CouchRest Changes should be used in a way that allows reprocessing all documents anyway. So this should not cause any problems but instead allows us to recover from the invalid seq.