Update ruby and bundler version

[matthijsgroen]

What

Explain what changes inside the code, this can be as simple or complicated as you want as long as it's clear

Why

Explain why these things are changes. This explanation is for your colleagues and your future self.

Code Review

Please consider the following checklist when reviewing this Pull Request.
More background and details here.

• Does the code actually solve the problem it was meant to solve?
• Is the code covered by unit tests? Integration tests?
• Does anything here need documentation? (Focus on why, not what.)
• Does any of this code deal with privacy sensitive information or affects security? Ask an additional reviewer.
• Is the code easy to understand and change in the future?
• Is the same code or concept duplicated? Find a balance between DRYness and readability.
• Does the code reasonably adhere to the Kabisa coding standards?
• Be kind.

[matthijsgroen]

CI is failing due to bundle install and native extensions... The container setup works, rspec tests inside the container succeed. Updated the readme with new instructions.

I want to keep this branch as is for now, to have as baseline while in a new branch I want to attempt an upgrade to Rails 6