Add support for Rails 6.0 and 6.1

[crigor]

This PR contains the following changes:

• update_attributes was changed to update. update_attributes was deprecated on Rails 6.0 and removed from Rails 6.1.
• For Rails 6.1 (but not 6.0), use :reflection instead of :association.
• ActiveRecord::MigrationContext.new has changed starting Rails 6.0 and it now requires 2 arguments.

Tests pass when running rake test:all for rails 5.0, 5.1, 5.2, 6.0, and 6.1. Rails 4.2 is currently failing though it's the same error as in master.

[Aelphaeis]

These changes were addressed first in #40 (6.0) and support for rails 6.1 was addressed in in #47.

Closing this pull request.