Fix Rubocop violations

[nard-tech]

In #3221, Travis CI failed due to Rubocop offenses.
In addition, violations ignored by .rubocop_todo.yml have been left alone long time.
I fixed these violations mainly using rubocop -a.

[nard-tech]

@mshibuya Travis CI failed in Ruby version 2.2 due to SyntaxError like this:

An error occurred while loading ./spec/controllers/rails_admin/application_controller_spec.rb.
Failure/Error: require 'rspec/rails'
SyntaxError:
  /home/travis/build/sferik/rails_admin/vendor/bundle/ruby/2.2.0/gems/rspec-rails-4.0.0/lib/rspec/rails/example/mailer_example_group.rb:25: syntax error, unexpected '.'
            options&.each { |key, value| default_u...
                     ^

Safe navigation operator with & is added since Ruby 2.3, so Ruby 2.2 raises SyntaxError.

Which is the best way to resolve this error?

• drop support for Ruby 2.2 from RailsAdmin
• specify another version of rspec-rails in Ruby 2.2

[camilohollanda]

I would vote to drop support for Ruby 2.2 from RailsAdmin

[mshibuya]

In the next major release of RailsAdmin I will drop support for Ruby older than 2.5.0. Please hold on until 3.0.0 development is ready.

[mshibuya]

The master branch is ready, can you rebase this to it?

[mshibuya]

Closing this because it will not be able to merged cleanly with the fix I made in the master 4379451 .
I'm sorry that I prioritized mine, but I needed it for 01e8d5f 🙇