Fixed a bug: Backbone is not defined.

When you use ```window.Backbone ``` javascript will use ```undefined```.  That way line 1276: ```if (!_.isUndefined(Backbone)) {``` will work. (https://github.com/forcedotcom/SalesforceMobileSDK-Shared/blob/master/libs/smartsync.js#L1276)

Or should this bug fixed on the _unstable_ branch?