Fixes to handling of cache bust query arg

When generating a jaggr request, the loader extension javascript will look for the cache bust string in the following locations:
* cachebust (case insensitive) query arg in window.location.href
* cachebust (case insenstive) query arg in URL used to load the loader extension javascript
* cb (case insensitive) query arg in URL used to load the loader extension javascript
* combo.cacheBust property in AMD loader config
* dojo.config.cacheBust property

The first value in the list above that is truthy will be used.