Use JSON API

**WIP - NOT READY FOR REVIEW/MERGE**

Closes site5/solusvm#55

* Newer versions of SolusVM support JSON responses, so v2.0.0 will use that over XML.
* Replaces [VCR](https://github.com/vcr/vcr) as a test stubbing library with [ShamRack](https://github.com/mdub/sham_rack)