[ALOY-1074] Enable developers to specify Backbone version in config.json

For https://jira.appcelerator.org/browse/ALOY-1074
Wouldn't it be better to not hard-code in our current shipping version? 

```
(compileConfig.backbone || CONST.DEFAULT_BACKBONE_VERSION),
```
(and set that to 1.1.2 in the constants.js file) That way, in the future we could just drop in a new version and update the constant.