only save the position when saving state

Before this change the entire Set object was serialized. Nothing but the
position in the migration list was actually read out in the load method.
This change will make it more obvious what you have to persist if you
provide your own State implementation.

lib/set.js

@@ -66,7 +66,9 @@ Set.prototype.addMigration = function(title, up, down){
 
 Set.prototype.save = function(fn){
   var self = this;
-  this.state.save(this, function (err) {
+  this.state.save({
+    pos: self.pos
+  }, function (err) {
     self.emit('save');
     fn(err);
   });