Fix test deprecations

[benjaminwols]

Minitest has new syntax which showed a lot of deprecation warnings in the test suite.
This PR converts to the new syntax.

[benjaminwols]

@maartenvanvliet I've updated the Readme badge to Github Actions and removed the deprecated code climate gem and updated Github actions to push coverage to code climate

[maartenvanvliet]

Can't say I'm a fan of the _(...).must_equal syntax, and would probably prefer plain assert_equal assertions over this. But the library already uses specs and this is the new way so 🤷

Thanks for fixing it all up!