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closes #7621

Let's change that to the new rubocop default, which is also the
suggestion in the ruby style guide.

See https://github.com/bbatsov/ruby-style-guide#percent-literal-braces

closes #7466

Use old style for Style/PercentLiteralDelimiters for now. Maybe we can
change this in the future, because the ruby style guide suggests to use
`[]` (which is also the new default for rubocop).

closes #7443#

See http://rubocop.readthedocs.io/en/latest/cops_rails/#railshttppositionalarguments.

It only applies to Rails >= 5

closes #7353

And update config for new/updated cops.

* ignore bin folder (generated files)
* allow same length for modules as for classes

closes #6635

closes #6271

* Add Rubocop
* Add guard-rubocop
* Enable Ruby linting on Hound