Make these tests pass and the computer will do serial commas for you!

test_serial_comma.rb

require "minitest/autorun"

class TestSerialComma < MiniTest::Unit::TestCase

  def possessives
    %w[hers his yours mine ours]
  end

  def test_possessives_1
    assert_equal "hers", serial_comma(places.slice(0, 1))
  end

  def test_possessives_2
    skip
    assert_equal "hers and his", serial_comma(possessives.slice(0, 2))
  end

  def test_possessives_3
    skip
    assert_equal "hers, his, and yours", serial_comma(possessives.slice(0, 3))
  end

  def test_possessives_4
    skip
    assert_equal "hers, his, yours, and mine", serial_comma(possessives.slice(0, 4))
  end

  def test_possessives_5
    skip
    assert_equal "hers, his, yours, mine, and ours", serial_comma(possessives)
  end

  def places
    %w[Seattle Lexington Athens Newcastle]
  end

  def test_places_1
    skip
    assert_equal "Seattle", serial_comma(places.slice(0, 1))
  end

  def test_places_2
    skip
    assert_equal "Seattle and Lexington", serial_comma(places.slice(0, 2))
  end

  def test_places_3
    skip
    assert_equal "Seattle, Lexington, and Athens", serial_comma(places.slice(0, 3))
  end

  def test_places_4
    skip
    assert_equal "Seattle, Lexington, Athens, and Newcastle", serial_comma(places)
  end
end

Fun times. I forked your gist and wrote a method to pass it. If you get a chance let me know what you would do differently.

PS. I updated your test slightly in my first revision. Let me know if I did that incorrectly.

@squaresurf. Yep, I totally had a copy/paste error on that test. I have fixed it!