Disallow instance variables in ActiveSupport::Concern#class_methods

[ytjmt]

Closes #5

Methods defined in ActiveSupport::Concern#class_methods blocks are used as class methods.
I added a rule to disallow instance variables in class_methods blocks.

module Example
  extend ActiveSupport::Concern

  class_methods do
    def test(params)
      @params = params
      ^^^^^^^ Avoid instance variables in class methods.
    end
  end
end

Cases using module ClassMethods can already be detected. ( covermymeds#31 )

module Example
  extend ActiveSupport::Concern

  module ClassMethods do
    def test(params)
      @params = params
      ^^^^^^^ Avoid instance variables in class methods.
    end
  end
end

[mikegee]

Thanks!