Modernize mbsy gem

#### Dependencies
  
- I removed old gems that are no longer maintained and gems that aren't needed by the test suite.
- I removed the debugging gems from the gemspec, put them in the Gemfile, and made them platform specific.
- I've updated all remaining gems to the newest stable release.

#### Tests

- I've rewritten most of the test suite to better unit test.
- base_spec actually tests the interaction with the API now
- ambassador_spec was very broken; I've replaced it with an approach that only tests the parameter validation and passing

#### Functionality

- Mbsy now understands all of the error conditions specified in the documentation (https://getambassador.com/api/#v2_response)
- Mbsy::Balance now has `#add` and `#deduct` methods to match the API docs
- The Rakefile uses the standard bundler require and specifies a default