fix update to work with redis cluster

the items do not have the same tag, so we must delete the entry from the
docsWithHistoryOps set outside the multi.

services/track-changes/app/coffee/RedisManager.coffee

@@ -21,12 +21,13 @@ module.exports = RedisManager =
 		# Delete all the updates which have been applied (exact match)
 		for update in docUpdates or []
 			multi.lrem Keys.uncompressedHistoryOps({doc_id}), 0, update
-		# It's ok to delete the doc_id from the set here. Even though the list
-		# of updates may not be empty, we will continue to process it until it is.
-		multi.srem Keys.docsWithHistoryOps({project_id}), doc_id
 		multi.exec (error, results) ->
 			return callback(error) if error?
-			callback null
+			# It's ok to delete the doc_id from the set here. Even though the list
+			# of updates may not be empty, we will continue to process it until it is.
+			rclient.srem Keys.docsWithHistoryOps({project_id}), doc_id, (error) ->
+				return callback(error) if error?
+				callback null
 
 	getDocIdsWithHistoryOps: (project_id, callback = (error, doc_ids) ->) ->
 		rclient.smembers Keys.docsWithHistoryOps({project_id}), callback