Updates for version 0.4.0, which is 1.0.0 but retains support for Rub…

…y >= 2.6.

This includes the use of `::JSON.generate` instead of `#to_json` and does not implicitly pick up ActiveSupport changes.

For #2.

Updates for release 0.3.3, which basically rolls back to 0.3.1 and ad…

…ds back support for Ruby >= 2.6.

This version uses `#to_json` and retains the behavior that ActiveSupport may override the `#to_json` behavior in some cases.

For #2

Finish 1.0.0

Finish 0.3.2

Finish 0.3.1

Finish 0.3.0

Finish 0.2.1

Finish 0.2.0

Finish 0.1.0