Update test suite to Minitest 5

Author: Josef Stribny

Gemfile

@@ -1,4 +1,4 @@
 source "https://rubygems.org"
 
 gem 'rake-compiler'
-gem 'minitest', :group => :development
+gem 'minitest', '>= 5.0.0', :group => :development

test/test_atomic.rb

@@ -13,7 +13,7 @@
 require 'minitest/autorun'
 require 'atomic'
 
-class TestAtomic < MiniTest::Test
+class TestAtomic < Minitest::Test
   def test_construct
     atomic = Atomic.new
     assert_equal nil, atomic.value