Update Rack Dependency, drop Ruby < 2.2.2

This update updates Rack to 2.0.8 to patch a security vulnerability as well as drops support for Ruby < 2.2.2.

Set api_base to production

Sets api_base to production environment. Can be overridden at runtime.

Customer List API

This version expands upon the CustomerList and Participant APIs to support adding new participants to your lists and managing existing ones.

Initial Pre-Release

This is the first development release and is not considered stable.