Adds rubocop and makes it green

The main idea here is to live by your guides, and do this to the extent that we make the tests fail if the style requirements are not met. This is as brutal as it gets in terms of applying a style guide to a project.

It will also ensure that we are on top of our style guide and that it matches our day to day work. Another idea would be to add the `.rubocop.yml` to the guides, and change both at the same time.

There are still a lot of changes to be done, most of which are in the rubocop todo. Maybe using rubocop auto-correct flag would be a good solution, since most are trivial changes.

*PS:* The ruby version was failing for me, because of the patch version. Until we find a less strict way of verifying ruby's version, I propose we just remove it.