chore(Ruby): add standardrb and format Ruby code

[jhult]

https://github.com/standardrb/standard

[jhult]

There is a PR open to add standardrb to Nixpkgs, but it is not yet merged.

Thus, my current attempt is to add standardrb via a gem. However, that fails with this error:

ERROR: Could not find a valid gem 'standardrb' (>= 0), here is why: 
Unable to download data from https://rubygems.org/
Gem::Net::OpenTimeout: Failed to open TCP connection to rubygems.org:443 (execution expired)

[robinbb]

I used 'Rubocop' to achieve same effect. Was available in recent nixpkgs.