Integer'ed in your face...

fibonacci2.rb

class Integer
  def fibonacci
    if self.kind_of? Integer 
      if self < 0
      "Must be a positive number"
        else 
          first_num = 0
          second_num = 1
        return self if (0..1).include? self
          (1..self).each do |counter|
          first_num, second_num = second_num, first_num + second_num
        end
        first_num
      end
      else
        "Must be an integer"
     end
   end
end

puts "'#{8.fibonacci}' should equal '21'"
puts "'#{1.fibonacci}' should be '1'"
puts "'#{2.fibonacci}' should be '1'"
puts "'#{0.fibonacci}' should be '0'"
puts "'#{5.fibonacci}' should equal '5'"
puts "'#{6.fibonacci}' should equal '8'"