Full refactor of all tests

[philnash]

This library previously had an entire Rails 4 application embedded in the test suite. This locked the tests to Rails 4, which is now no longer maintained and was raising security issues on the repo.

This PR revamps the entire test suite with the following changes:

• Uses Appraisal to test against multiple versions of Rails (5.2 and 6 now)
• Uses Combustion to embed a minimal Rails application in the tests, keeping most Rails specific files out of the spec directory.
• Tests the generators using Rails generator tests instead of shelling out and testing the result
• Includes 100% test coverage of the library across helpers, controllers and models
• Has minimal changes to the existing code and nothing breaking (though we are now in a position to start changing things with a bit more confidence).
• Adds some more documentation to the gem (it took me a long while to work out how the passwords_controller override worked)

This should be a minor update to the actual library and result in no changes to any user code.

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• [x] I acknowledge that all my contributions will be made under the project's license.

[philnash]

Thanks for the review @senekis!