Apply rubocop

I started with rubocop -a, and then picked an order that seemed
appropriate for applying "unsafe" changes through rubocop -A --only [...].

I also edited a bit rubocop's configuration to make sure the cops are
compatible with ruby 2.

.rubocop.yml

 ---
 Lint/UnusedBlockArgument:
   Severity: convention
+Style/HashSyntax:
+  # FIXME: change this when we enforce usage of ruby 3.
+  EnforcedShorthandSyntax: never