Force ministest gem version

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Gemfile

@@ -13,5 +13,5 @@ group :development, :test do
   gem 'wikicloth', '~> 0.8.0'
   gem 'minitest-reporters'
   gem 'nokogiri-diff', '~> 0.1.2'
-  gem "minitest"
+  gem "minitest", "~> 4.0"
 end