This is a little too picky, even for us.

We have to use the lowest common denominator to check the supported syntax
and in our case it is Ruby 2.1.
Please note that it will not help with unsupported syntax in HAML files
because they are not checked by Rubocop.

`Style/VariableNumber` is explicitly disabled because I don't think we
care if we name a variable `var_1` or `var1`.

This reverts commit 70faf5fd, reversing
changes made to 2307eb84.

Avoid multi-line ?: (the ternary operator). Use if/unless instead.

See #17478

This enables following cops:

Check for useless access modifiers

  Lint/UselessAccessModifier

Checks for attempts to use `private` or `protected` to set the
visibility of a class method, which does not work.

  Lint/IneffectiveAccessModifier

This also disables two false possitives in concerns.

https://github.com/bbatsov/rubocop/blob/v0.41.2/CHANGELOG.md