Ruby unittest error: "you should require 'minitest/autorun' instead."

I have a very simple unit test that I copied from http://ift.tt/17O7Bdn.

require_relative "simple_number"
require "test/unit"

class TestSimpleNumber < Test::Unit::TestCase

    def test_simple
        assert_equal(4, SimpleNumber.new(2).add(2) )
        assert_equal(6, SimpleNumber.new(2).multiply(3) )
    end

end

Executing this code causes error:

Warning: you should require 'minitest/autorun' instead.
Warning: or add 'gem "minitest"' before 'require "minitest/autorun"'
From:
  /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/test/unit.rb:3:in `<top (required)>'
  tc_simple_number.rb:5:in `<main>'
MiniTest::Unit::TestCase is now Minitest::Test. From /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/test/unit/testcase.rb:8:in `<module:Unit>'
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/test/unit.rb:670:in `<class:Runner>': undefined method `_run_suite' for class `Test::Unit::Runner' (NameError)
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/test/unit.rb:255:in `<module:Unit>'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/test/unit.rb:9:in `<module:Test>'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/test/unit.rb:8:in `<top (required)>'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from tc_simple_number.rb:5:in `<main>'

It's a little bit weird, because I remember it worked fine before; then I installed some packages that might cause this trouble.

What might be wrong?