Use self to declare class methods

We prefer using self.method_name instead of the class << self way of defining class methods.

class Apple
  def is_green?
  end

  class << self
    def all_apples
    end
  end

  def another_method
  end
end

When reading this class with the number of methods defined, it's hard to tell the difference between a class method and an instance method when the methods are using class << self.

class Apple
  def is_green?
  end

  def self.all_apples
  end

  def another_method
  end
end

Here, the class method is clearly defined and there is no ambiguity when reading this class. Moreover, it is now easier to refactor and maintain.