applications.js

if (Meteor._reload) {
    Meteor._reload.onMigrate('session', function() {
        return [true, {
            keys : []
        }];
    });

    (function() {
        var migrationData = Meteor._reload.migrationData('session');
        if (migrationData && migrationData.keys) {
            Session.keys = migrationData.keys;
        }
    })();
}