Drop support for Rails 3.0
Configure RuboCop and fix most offenses
Set up unit tests
Remove useless require
Use RSpec for unit tests
Add more specs
Make SchemaLoader a class

- Improve testability by injecting dependencies
- Store configuration in an instance variable to avoid re-fetching it
  during the same call to init_schema
Avoid duplication
Settle on using parentheses in method definitions
Enable recommended RSpec settings
Configure and fix RuboCop offenses
Clarify code
Use Rails' load_schema to load the schema
Restore compatibility with Rails < 4.1

Older versions of Rails do not provide an API for loading the schema
file. By putting the schema loading in a separate module, the same
interface can be used.