Remove multi_json as dependency in favor of std-lib json

[otherguy]

This PR is almost identical to #212 but for security reasons we want to maintain our own fork.

CI is also updated to include newer Ruby and Rails versions (currently failing on edge but that is expected).

[byroot]

Looks fine, but lots of issues with CI. Let's get the MultiJson removal and CI fixes in different PRs.

[byroot]

I merged the dependency removal.

However I tried to really update the CI matrix but it's a huge pain -_-.

[otherguy]

@byroot thank you for resolving this with 536716a :)

As for the CI matrix, perhaps you can add 7.2 and make failures of edge just warnings?

[byroot]

The failure on edge is also on 7.2. I dug a bit as to what is going on, but I'm a bit tired.

[otherguy]

@byroot can I help you get this released as a new version? What's missing?

[byroot]

The CI would need to be fixed, tests are no longer passing against Rails edge because of some change in the router.

I know the gem works fine, it's just a test helper that was reaching into Action Dispatch internals that need to be refactored/updated.