v0.5.0

Drop support for Rails 5.1 or earlier
Add tests for Rails 5.2, 6.0, and Rails master
Use standardrb - @excid3
Refactor controller extensions into modules - @excid3
Add subdomain_lookup option to change which subdomain is used - @excid3
Unsaved tenant records will now return no records. #227 - @excid3
Refactor test suite and use dummy Rails app - @excid3
Remove tenant getter override. Fixes caching issues with association. - @bernardeli