Avoid bundling minitest that requires ruby ~> 2.2 for testing rails 4.2.x

This should fix the CI failure https://travis-ci.org/github/rails/rails-dom-testing/builds/661676901

Author: amatsuda

gemfiles/active_support_4.2.gemfile

@@ -3,5 +3,6 @@ source "https://rubygems.org"
 gem "activesupport", "~> 4.2.0"
 
 gem "nokogiri", "~> 1.6.0"
+gem 'minitest', '< 5.12.0'
 
 gemspec path: "../"