Spot API

* Spot API now lives in the wrapper gem instead of in engine code
* Added an exception type (ExpectedResponseFieldMissing) for when API responses have changed on us
* 100% test coverage
* reformatted meta key hash in Cb::Utils::Api for maximum sexiness
* lazy loaded and cached Spot client's instance of Cb::Utils::Api 
* .ruby-version hinting file to lock ruby's version in the project for those that use ruby version managers